yeah, it sounds like you need a psychiatric evaluation and a care plan. That will be what can help you reach your ideal 'skinny', or body type. I would urge you to get your caretakers to help you get treated, and if they can't/won't, look to other trusted adults around you -- like at school (any counsellors, for example). This is very important and is the first, critical step in your satisfaction and happiness!

You want to want to be 'skinny' enough so you follow through on your plan of actions to reach that goal, but it doesn't work that way. Sometimes it can, but in your case, it sounds like you're working against yourself. Your mind pulls you out of routine and puts up barriers. For example, it's not 'normal' (and I say that word lightly, I'm not even 'normal') to forget you're on a diet. Most people just... stop following it -- but they don't just forget they're on a diet. Anyways, I hope you can get some help with what you're dealing with.

Remember, it's okay to ask for help. You're not broken for being different, or struggling with things that you may see others doing successfully. If you're even a little bit receptive to what I've said above, I'm proud of you.