Wow,..you are getting your ass ridden a little bit here.
Have you dealt with the obvious ? You were ok to share him, as you felt like you also had a piece of his attention, time and feelings. Another woman was not getting, what you didn`t already have.
When you don`t have those things anymore, but someone else does,..jealousy can rear its ugly head.
You have feelings that you haven`t come to terms with yet. Its hurtful to see someone else get what you still desire. More hurtful when you know them both, and can see what they get from each other.
So,..how to deal,...when you feel jealousy rise, you have to remind yourself where it is coming from. From the feeling of loss. Your heart still wants something. You are logically trying to tell it to 'get over it' but that won`t work, if you haven't identified the pain you are feeling.
Did either of you ever get a chance to figure out what went wrong ? Or did you both just blow, and walk separate ways ? Unresolved feelings have a way of festering.
It might be in your best interest to have some type of talk with him, lay cards out on the table about what went wrong. Anything awkward, needs some clarity.
He may or may not go along with this. This is where you can only control yourself. If he doesn`t want to participate, you can then know the score. Somebody feeling selfish, or who has already moved on, might not want to babble.
Thats ok, at least you know you tried, and that it is really over. Maybe not a perfect resolution, but at least you have a point as to 'get over it' from.