Your letter shows you to be a remarkably insightful person for your age. You have put a lot of thought into the events that have shaped your life and what changes you would like to make as you launch into adulthood. Please give yourself a break. As a child you did what you had to do to get along and to please the big people who were in charge. Hiding emotions and acting more mature than you probably felt kept you safe. My guess is that you developed an eating disorder in order to feel like you were in control of at least something. It all makes a kind of sense.
Fortunately, we are not stuck with continuing the behaviors that were necessary in childhood. It takes effort, but people can give themselves a “personality transplatnt” if they want it badly enough. You’ve already found that the very tactics you used to stay safe as a child are getting in your way now. It sounds like you’re ready to confront your difficulties and make some changes.
One thing you can do is act “as if.” You are an excellent observer of people. Use that skill to watch people you think are more like the way you want to be. Then do a little role playing. Act as if you are like them. Believe it or not, by making a game of it and acting “as if” over time, you will start to incorporate those new behaviors. The song “Whistle a Happy Tune” is a powerful and fun statement about the usefulness of this technique. You can find several versions on Youtube. Let it be your theme song for awhile.
You might also find it very helpful to get involved with some group therapy. An effective therapy group gives its members a safe place in which to try out new behaviors and to get feedback from the other people. Rehearsing how you want to be in the group will help you get comfortable with sharing more of yourself.
I realize you are impatient with how you are feeling. Harness that impatience and direct your energy toward doing a little every day to make change. It will be difficult but, judging by your letter, you have the intelligence and motivation to try.
I wish you well.