I cheated on my ex-boyfriend with my current boyfriend. Now he's insecure and thinks I'll do the same to him?

Well that is really hard to do. That is the side effect of starting a relationship through cheating. With your independent streak, it is probably going to be hard to quell his jealousy. If you come to the conclusion that it might now work out, do a clean break and start afresh, not cheat on the side again then jump ship.
This is also payback to him since he was part of the cheating in the first place. You have to lay it out to him that you both had your hands in the cheating business so its not like he is the only person to feel insecure. How do you know he will not be stealing another girl again.
What needs to be done is to sit down and talk the hard and difficult talk that what happened was a one time thing and that he is all that you want right now. That you promise and give him a guarantee that there is not going to be a repeat. It was a one time bone headed mistake!
There is nothing more you can really do to convince him. And since it is not realistic for you not to see or be friends with male colleagues, if he won't believe you then splits ville sooner rather than later may be the best solution for the long term peace for both of you.
its the cold hard truth.
Trust is hard thing to build and doubt could be even harder to quell.

That is a tough one. Let me tell you that I was in a situation exactly like yours except it was with a girl who was married. It started out as some flirting and when she moved into her own apartment we started seeing each other even though she was still married. Long story short she ended up doing the exact same thing again. She moved out and started see some guy from her work. So I have been on both sides of the situation. Let me tell you that from a guys prospective I always believed that if she cheated on her last husband what is going to keep her from cheating on me. The answer was nothing. She just ended up cheating on me and we are in the middle of a divorce. So the real problem might lie with you. I am not convinced that things will change with you. However if the two of you are both willing to keep things open and talk about his and you feelings you might have a chance. It is going to be an uphill battle the entire way, so you might want to ask yourself right now if it is worth it to you.

Never give up your friends for a lover, giving up exes is one thing but friends matter in the end. Talk to him and make sure he knows that your friends matter to you, because if he were to leave then they are all you have to support you. To change his fear of being cheated on however maybe you should do a series of things for him to show that you are in it for the long haul. Things that don't have to be sexual cause then he may think you are just covering up the problem with sex. I mean like a random card for him at work, a dinner for both of you, then a random picnic, and maybe a just some nice words here and there like, 'You know I am yours always no matter where I am.' Eventually he should feel better about you going away and not worrying about who your with and where you are at.

this is why cheating never pays in the end. It always comes back to bite you. Have you ever been cheated on ? It really hurts and you shouldn't have done it. BUT, if your man is insecure because he thinks since you cheated to be with him, then you will cheat ON him, then remind him that he was a part of it too, he's just as guilty. If he is acting more jealous now than he did before, just remember the guilty dog usually barks first, maybe he's cheating on you ? My ex accused me of everything under the sun, and I never did him wrong, ever.it turns out he was cheating on me on a regular basis.