Your feelings to be jealous are or better yet "offended" that he has expressed it to you ..might be worth jumping the fence right here. He is useless.
What, you get to cherrypick your emotions?
If so, a good rule of thumb is to never pick the damaging ones.
If you can honestly mind read, you have a wonderful career with the CIA. If you can't, you don't really know if he cares more for his ex than you.
Jealousy is about you, not him. It indicates that you are insecure about the relationship. If he is giving you reason to be insecure, you need to talk to him about the things he is doing to make you feel insecure. If he refuses to change what he is doing, find someone else. If he isn't giving you cause, you need to examine your personal insecurities.
Relationship are tough. They take work. But above all, you need to be true to yourself, because when the boyfriend is gone (considering your age, he probably will be at some point) you are all you have.
No. You can't control how he feels, and you don't technically know how he feels on the inside about it.
So just let it slide. You can wish to understand people, but when you take how they feel personally, you're going too far.
No, why be jealous. Its almost as if your a rebound. Thats not a good thing, because she can get inside his head anyway she feels like it. (something is wrong with that but its nothing to be jealous about)