Okay my two cents.

Trust is given not earnt. If you give your trust willingly to him again it has to be your decision. Nothing he can say will earn your trust back (in my opinion)...

My ex (yes EX) cheated on me, he told me I was irrational and imagining things when I confronted him - thus causing me to doubt my own intution and mind. I thought I was going crazy. Then after I had proof he 'fessed up. This was a guy who I felt safe with, who I never expected to cheat, who was my world at the time. We had been together for almost 6 years. The woman (girl??) in question was the receptionist at his work, he had spoken of her often and I never gave it another thought.
I hope that you can get through this in the way that will make you happy in yourself. Hugs to you xxx