It certainly COULD be a phobia. Or, it could just be you got hurt by a jerk and it is taking you time to completely get over that. So, let me start off by saying this....
Your ex was a jerk. He didn't deserve you. I am so very sorry to hear this happened to you.... but there ARE good guys out there. There are guys out there who would NEVER cheat on their partner. You may very well have found one. So, for your own sake, PLEASE do not let that jerk from your past ruin your future.
Now, with that said, I don't mean to over-simplify it like it is just as simple as you saying "Okay. Cool. I'm going to trust my boyfriend now." I do understand that may be hard for you.... but in time, if he never gives you any reason to doubt him, it will start to be easier and easier for you to trust him again. Of course you are freaking out and half-expecting he'll just turn out to be the same kind of jerk. You once thought you were in love until the person you thought you loved betrayed your trust in the deepest way possible. So, he's now trained you to think that happiness cannot last.
Of course it is going to be hard to trust again after an experience like that. ....But it will be easier in time, especially if you are with somebody who can be trusted. I will say this, though... if you continue to find it too hard to trust your new boyfriend even though he has proven himself very trustworthy.... please don't hesitate to seek out counseling. Too often people avoid that because they seem to have this stigma about it. It is NOT a sign of weakness to accept help when you need it. In fact, it shows great strength even just to be able to admit you have a problem.
If you can't seem to get through it on your own, there are trained professionals (therapists, psychologists, etc.) you could possibly work with who could help you get there so much faster. So, if you think that is something yo:u. You DO deserve somebody who will appreciate you the way you deserve. Being able to believe that is the first step. You DO NOT deserve to be treated the way your ex treated you. You will find the right guy some day... and that may even be the guy you are with now. So, don't let your past ruin the chance for that.