You both have a history together and that is what brings you back to each other it a familiarity that he feels that you are safe and do not expect anything from him but is this kind of relationship making you happy? No, it is not it is making you question yourself about why he won't commit to you when you have been there for him longer than any of his relationships have lasted. He may not want a relationship with you because of hurting you and that he thinks this way is much safer. It good that you wrote the letter and sent it to him this way he knows exactly how you feel but since you haven't heard from him since sending the letter it may be time to cut all ties in order for you to heal from this and show him that you will not be used as an object. If you need to avoid him at any cost then you do that but if I were you I would show him that your life will go on without him the next time you go to another event make sure you have a date that way you won't get the urge to give into him and if he calls avoid his phone calls this it he only way you will be able to give over him completely.
He will continue to use you as long as you allow him to and until he respects you as a woman and someone he has known for many years he will never truly be yours, maybe if you show him this he will realize what he has/had but I wouldn't wait for him any longer 12 years is long enough for him to know if he wants you as his girlfriend or not.
He must be scared that he isn't right for you since he called you he must feel something for you or he would have left it go. Sometimes men act a certain way because they have learned from their parents it's a learned behavior if his parents didn't show much affection then that could be why he cannot bring himself to give you a chance or maybe he has been hurt in the past and doesn't want to get hurt or doesn't want to hurt you since you are friends. I can't really tell you exactly why he did what he did the only way you will get answers is to ask him and hope that he will open up to you. If I were you I would make a date with him and during that date ask him the things you want to, he may have been waiting for you to say the word and since you didn't say anything that night at his house he got scared that he wasn't saying or doing the right things. This may be why he felt he couldn't breath and needed air he was panicked, do you know if he has a history of panic attacks? You really need to meet with him face to face to get the answer you need and deserve.
Ask friend about his past relationships and his parents this may give you insight as to why he cannot commit to you or anyone else for that matter. You may have to take action as far as a relationship with him is concerned he seems to be dragging his feet or scared of commitment period.