Does he still love her?

Your questions are "Can I trust him or should I break up with him before I get hurt"

I think you have already answered the 'can I trust him' question. Because you whole post tells us that you do not. And if he is openly flirting with an old girlfriend in front of you. Have you asked him to stop doing that? He should be sensitive to your feelings. If he refuses to understand how him hanging around and flirting with an old girl friend might threaten you. Then you have to ask yourself IS this the type of BF you want.

Now, "Should you break up with him before you get hurt?" This is a game we all play with ourselves. Because if you broke up with him now, you already would be hurt. Wouldn't you? So, in that sense you'd save yourself nothing.

Some girls like to do the breaking up rather than the being broken up with. Why, I don't know. I'd rather give my all, love somebody 100%. Then if we broke up, I could walk away knowing I'd loved him 100%—-and it wasn't good enough. Knowing that would help me.

Hope something in this answer helps you. Blessings and good luck