You know if I were you I'd try to take one step at a time. If you are under a lot of stress with your separation then I'd wait until I'm completely freed from stress, mental discomfort and problems caused by a separation. Remember you and your husband lasted for 15 years...not 3 months. You are still hurt..a separation is not an easy or nice experience to go through.
Once you get yourself together, feel happy, not confused and you feel confident one more time then you should start thinking of living those moments you missed or adventures you couldn't see yourself breathing when you were married.
Furthermore, a lot of people could get hurt with this frienship of yours. End it. You are not even feeling the guy anymore. Life is full of great moments, men and surprises. If you'll end up hurting people then this friendship is not worth putting time into. Your time to live life once again will come eventually. You'll know the time and person is right when hurting others is not an included factor.
Good luck.
Sounds like it was a rebound thing. If alot of people would get hurt by this relationship, then maybe you should end it now. Sounds like your heart is telling you the same thing.