If they did something wrong I wouldn't bring attention to it, it's definitely not the place as with a letter like this, you shouldn't try to start a fight as it will only distract from the intended focus. However, I just wouldn't lie and say they didn't do something wrong, it's probably best to just not say anything and not assign blame.
I also wouldn't seek a sample letter or even suggest anything specific to you because these are your parents and they know you. If you start spouting off specific phrases it's going to sound less like you and that's not going to reach them in the same way. Also, what you're feeling is your own feelings and getting that written down is only something you can do.
Just write from the heart. Let them know what's happening in a clear and concise manner. Don't ramble but get what needs to be said written down. But maybe leave it open, don't write it with aggression. Leaving it more informative and open will give them that chance to accept you and maybe even one day support you if and when you want that support.
Just write as you, and avoid lying. You can embellish if you wish to, like if you do have love for your parents somewhere inside, you don't need to specify that you kinda love them, you can embellish that love if you find it's relevant to the letter. Make it clear to them that this is your life, again, not in an aggressive manner, just in a way that they know you can't be simply talked out of it.
Best of luck!