I think it would be a much harder blow for him to leave for a woman than a man because YOU are a woman, too, so that may be what you’re experiencing as well. You didn’t have to deal with that issue before because he was dating men and you’re not a man, so like you said, that’s not something you could be for him. Now that he’s dating and interested in women again, though, you’re probably feeling more of a rejection from it because now he’s dating what you are but not wholely interested in being with just you.  The fact that you aren’t really interested in a relationship with him will likely help you get over this more quickly since it’s not that you want to be with him now… it’s just that your ego doesn’t like the thought of him not choosing you as you are a women and now he’s interested in women.  Does that makes sense?  You just have to rectify those thoughts in your head, and it sounds like you’re already almost there, recognizing that it’s your PAST self (or ego) that is feeling upset and not you with what your situation with him is right now.  He didn’t really reject you personally, it’s just very clear that he doesn’t know what he wants and hasn’t for a long time now. You are very clearly still a very important person to him.

I agree that working on setting boundaries is important, too. It can be as simple as just setting the boundary to keep the relationship strictly a friendship… only do what friends do… hang out with each other, no kissing, no cuddling, no sex, but being emotionally intimate is fine (as in being a support system for each other). If you feel yourself developing romantic feelings, just back off from hanging out with him a bit. Dating someone else should help with this because lots of times we can feel romantic-like feelings for people we actually aren’t interested in or a match with just because we’re feeling lonely and want to feel that attachment/comfort/security… which could possibly be a reason why you’re holding onto the idea of possibly being together now… you haven’t met the right person yet, so he still seems like a good candidate. When the right one does come along, those feelings will likely disappear altogether.