Ok my first thought would be that you need to try and vent some of your anger-go for a run or put some loud music on or have a nap (that always makes things less angry/upsetting for me). Then think about it after you have calmed down a bit.
When you're calm, talk to him. Tell him how insecure this girl makes you feel and how it upsets you. Tell him that you want to spend time with him.
Well done for not stopping him going out, that's admirable considering the situation. I would be insanely jealous and insecure about that girl so i think you're coping really well. But in the end, if you truly trust your bf, then you need to try and relax about him seeing her if there's no way of avoiding it. And make sure he spends more time with you than he does with her-that's only fair (girlfriends rights!).