I quit dating all together. Someday I will again, but the majority of "relationships" if that is what you can even call them just stink. We have a shortage of honest, caring, loving and responsible people out there. Not just men, but women too. It breaks out spirits and makes it hard for the good people to trust anyone.
Since you are already with this guy and as long as you do like him and are attracted to him then go ahead and take it slow. He seems very understanding, so I would stay with him as long as you can. It may take you years to trust again. I know that probably doesn't make you feel better but it is true.
But, if it doesn't work out with this guy then after him I highly recommend waiting a while. Or until you find someone that you do fully trust.
Don't take this as a weakness, take it as a strength. That is what it is. Most people would just put up with the lying and cheating, you are one of the few left that wont. Once a cheater always a cheater and that is not a lie. If you let them get away with it once, they know they can get away with it in the future.
Just continue respecting yourself, take things slow and try to trust, but not blindly. You still need to pay attention. Take care of yourself.