I need to remove this from my every thought first. Since you've asked for advice on a particular issue, rather than on your relationship, I won't get into how immature and irresponsible your whole story sounds. As for forgetting this woman your boyfriend slept with, well, I imagine you can't forget about her because you don't trust him and you haven't forgiven him. It sounds like you've made the attempt, excusing his behavior because you were aware of his playboy nature - so you have to figure out how to let go, and nagging him or becoming dependent on his constant proclamations of love for you will not help. I suggest looking deep into your feelings and thinking about whether or not you have a problem with this woman and the past or with your present situation and the risks of the future.
Cold feet are much less troubling if attributed to the trivial than if firmly grounded in reality.