And you shouldn't "fall back and just believe him" You've not only been betrayed, deceived and cheated on once but many times. One time is enough for most people to have trust issues with their SO, how much more for someone that has faced it multiple times?

Personally, I would have written him off by now but I can understand maybe wanting to work it out with him. Since that's the case I would say only this. It will be difficult at best to get over the past hurts, without counseling, I think for both of you as a couple you need this. At the very least you should seek counseling but I do feel he needs it too. Without any outside help, tbh, I am pretty sure it would be the same old pattern for you two.

On his end, IMO, he should be repentant of the offenses against you. What I mean is, has he truly come to a place where he wants to be faithful to you? Has he shown this? As the offender, it's up to him to give you a reason to put your trust in him again, to earn it from you. If it's too easy for him to get you back there's no consequence for his actions and I doubt it will ever change. You can't make him do anything but I say all this just to say you should ask yourself these questions.

Hope this helps, sorry you're going through this
*hugs*