Well, you say you have been together with your boyfriend for a year now, so I'm going to ask a straight question.
Do you have any reason NOT to trust him ?
Has he cheated on you, lied to you, or similar.
If he hasn't, then I think that you re just insecure about yourself, try not to think thougths that make you feel insecure, don't bother comparing yourself to other girls, or even try to paint up diffrent scenarios.
It will just make things worse.
However, you can ask your boyfriend where he has been, and what he has done etc, always, listen to him, what he has to tell you, that can make a big diffrence.
I'm talking out of personal experience with my girlfriend, that has been, and still is to some degree jelous of other girls, even friends that I've been knowing for 10 years or more.
You also said that he is trying to rebuild a friendly relatioinship with a girl that he has known for a long time, and that he asked out.
Well you seem to know the story there, so why bother thinking those thoughts at all ?
Why not focus that energy into supporting him, and hoping that he will be a good friend once again .
Your last sentence, kinda made me think, and that's probably exactly what my girlfriend tried to do.
"I just don't think I can compete. ;__;"
NEVER compete, you re not there to compete, you re there to make your life better and more fun to live, and he is there for you !
It's a good thing that he doesn't drink! Be happy about that !, Seems like he has some standards that he isn't going to give up.
Finnaly when it comes to the flirting, you could actually talk to him about that, tell him how you feel when he does that infront of you, but also have in mind that you might be more sensitive than before and that some things that you didn't care about then, suddenly matter now. Try to talk to him about it, and see what he has to say.
So be cool, take it easy, and try to communicate more with your boyfriend, tell him how you feel, but also listen to what he has to say, and how he sees things, it helps a lot!
Also remember, you have a value! and you DESERVE to be loved, and cared for, no matter how other girls are, or how they look, what they do!
You re special, and that's probably why your boyfriend choose you, and not someone else .)