How should me and boyfriend get over trust issues?

I have trouble trusting him because in the past he's gotten a little to flirtatious while he talks to girls through the phone, and to me that's disrespectful and not trustworthy to even flirt while in a relationship, and I have also done the same out of spite because he has. I also while we were broken up, kissed another man but he considered it betrayal and "cheating" how do we get past this? Well, I guess how do I trust him to talk to other girls when all he used to do is "Flirt"