sorry to be joining the thread late - but i have had similar feelings after breaking up with a long time S.O.

I think you have to really sort out some fundamentals:
1. When you were broken up, and he had a "date" - were you jealous because you wish you were with him, or did you feel bad because you were alone and you did not have a date.
2. Were you heartbroken at the thought of being without him, or heartbroken at the thought of being alone?
3. Did the problems you had that led you to break up with him go away? (They might have, if you began to communicate more after the breakup) ... Or do you think the same problems will arise again?

My experience is:
1. It is nearly impossible to remain close friends (esp best friends) after a breakup. You either get back together or you lose touch. To stay in limbo only plays mind games with each other and prevents either from getting into a different healthy relationship. You have to be willing to cut ties if you are going to break up again.
2. Watching an x get a S.O. before you is always hard. There are alot of feelings of emptiness that goes with that. But that is not the same as wanting your x back for good reasons. You have to get away from that - which is a good reason to cut ties a bit.
3. Being on your own for a while is always a good option - even if it is often the most difficult option. You have to be self sufficient and like yourself before you can get in a relationship - whether it is with your x or in a new relationship. Otherwise you will start to define yourself and your happiness by the relationship you are in. You have to be happy with yourself first. It is good to get a taste of independence for a while.

I am sure things will work out for the best with you, as you seem to be thinking about the issues at hand. And you have ALOT of time on your side - so don't feel compelled to stay with somebody when you are not optimistic about the future. But if you can work out the issues and things look bright, then good job too, for working through the rough times. Your heart will guide you.