As a person trained in safeguarding vulnerable adults and young people in the uk, I only know the human rights, protection and safeguarding laws and procedures for my country. However Canada has a good reputation for human rights and I'm sure the two countries processes must have more similarities than differences.
There are two safeguarding issues I can identify from your writing, that if I was in a position of trust, I would have a duty to report. The first is the obvious one, that you mum is concerned about. She is worried that you are thinking about younger females in an unhealthy way, which if not resolved could impact your life and others in very unpleasant and injurious ways, if not resolved. The second safeguarding issue is that you have human rights to privacy, to development your own sense of sexuality and to have a safe, private and protected personal life. If you were in the UK, your Mum would not be able to make you go to counselling unless it was deemed to be in your "best interest". As you are a minor and have not committed a crime, (and even if you had) all adult in a position of trust involved with your care and support, including any legal guardians/family/parents and counsellors, would have to consider your views, beliefs and preferences, respect privacy and safeguard your personal freedoms. Adults, whether under care or not in the U.K. and with mental capacity have the right to make eccentric, seemingly unwise decisions. If you do want to be in the police, you will also be in certain situations, a person of trust for vulnerable adults and minors, so you might think about starting to learn your country's human right laws, now.
The questions you might consider about your mums attitude are:- How educated and competent is she in general young person upbringing? It is difficult to be objective about your own family until you have something to compare it by, so you will have to think deep and levelheadedly, observe, compare and ask your friends carefully about their personal and family standards. How good, safe and informed was your mums upbringing? How much support does she get in raising a young person? From my personal experience I can tell you, A noisy opinionated mum is always going to be a noisy opinionated mum, no matter how old you get, and you are likely going to have to learn to appreciate her in that way. My Mum was brought up in the depression after the war. She was systematically neglected, bullied and put down for being a girl my her own mother and I think bullied and possibly sexual assaulted by her brother. She is a loving kind and extremely sharp/clever person but an emotional intelligent mother she has never been. It took me till I was nearly thirty to understand her (I've always loved her, she is very lovable. I also always know she was extremely hurt and still hurting.) and in the meantime I had to learn things she couldn't teach by myself. I'm still learning.
Also,the thing you must remember about mine and your mothers generation and those older, is that we simply did not have the good access to intelligent information as you do. A lot of things we had to stay confused about or learn the hard way e.g. thought bad unhealthy experience, instead of simply looking quality advice up on google. University was a big eye opener for me, I was socially naive and had to deal with serious issues more or less alone. I made mistakes and have had to alter my whole life because of crap events that happened. In short, I did not get out of a very bad boyfriend situation quick enough. If I had known the things I do now about other people (more deviant than myself ) and my own capabilities of dealing with people. I would have chosen a different more rural University better suited to my sensitive/almost vulnerable nature and I would have been a stronger healthier person now.
Decisions and personal attitudes of young people can have a lasting impact on their life and other people lives, longer lasting even than the original attitude or belief in a decision correctness. Sometimes by over sexualising a person or a type of person, you can miss and limit your own experience of them as individuals. Also, some people because of bad experience will not let others see their true sexuality, so you will never fully be able to comprehend them in that way. If you are a person of trust with limited comprehension of the personal values/cultural law standards you will be employed to enforce and individual people's personal needs for safety and reassurance you could miss clues that could help you protect them. In other words by not assessing your own motivations and values well, you could become a very bad/poor cop.
Your mother may have become aware of your own naivety and feels ill equipped to deal with it herself. It is good that you are learning about yourself now and you could extend that to learning about other people. There are some really great book on Amazon for people your age about critical thinking, developing healthy attitudes to relationships and sexuality, advance learning techniques to ace study and exams with least amount of effort and how to build confidence. I would have literally and unashamedly killed to have got my hands on that kind of intelligence at your age. Maybe if you found a few books of personal relevance and asked your mum for them, she might see that you are trying to learn about yourself responsibly and cut you some slack.
In regards to counselling and dreams, I've just started an introduction to art therapy course and I have learnt that even Jung thought that dreams were personal and can only be interpreted by the dreamer. In art therapy the therapist, these days, no longer makes analysis of clients dreams. They only get the person to talk about their dreams if they want to. I hope you have a good councillor, but if you don't, there are still things you can do for yourself. In your dream journey, after you have written the details of your dream, you could also write about what you think the dream was about, what it meant to you and how you think that would differ from social acceptances. You could also rewrite the dream so that it is more pleasing to you. I find when I do that, my dreams start to become more like the ones I most like to enjoy. It also helps in gaining a better understanding of what you really like, (or in sexual terms, what really turns you on). It might not be that you prefer younger people. It might just mean you like being the more assertive partner. That, doesn't have to be a bad thing at all and for some people that would make you highly attractive. In terms of being a police officer, I would think being able to make quick assertive and authoritative decisions would be an advantage, so long as it was applied with diligent critical self reflection. Strong and fair assertiveness is something you could enjoy blamelessly developing in your personal life also.
I don't think you fucked up. I think you are probably just very "healthy". I did, at your age have sexual dreams about cousins also. I didn't write them down cause I thought they were a bit too gross. I did draw extremely naughty pictures of other kinky fantasy though, which if they had been discovered, would have been highly embarrassing. One of the advantages of being a weirdly dyslexic kid is that I naturally set personal boundaries from a very early age. To this day, no one gets to see my "dirt" unless I think it might be fun/interesting to show them. There used to be shouty wars, tantrums and days long stand offs if my mum dared to come anywhere near my "art desk" with a black bin bag, (spent felt tipped pens and scribbles are highly valuable and useful items to some people, you know?) For a normally placid, polite person I can be alarmingly stubborn and defensive of my personal shit and as you get older I'm certain will come across an amazingly lot of other people like that. Infringement of personal shit is what starts a lot of wars, I think.
Lastly, I don't think you should beat your self up about having untidy thoughts that your mum got access too. All the best thinkers have lots of untidy thoughts. Most just get better at hiding the more unacceptable ones. You only have to look at this link about untidy desks to see that successful people aren't always the tidiest thinkers. http://computhink.com/10-messy-desks-successful-people/ Successful people challenge themselves and society. Learning how to set respect boundaries and when it is reasonable or beneficial to break them is an on-going thing because society is forever altering attitudes. However, you should have and progress some personal privacy safeguards and standards to protect yourself and the individual you live with.
Hope this helps a little and that things are working out for you.
Anyhow, Kind Regards. I hope to see you about...