I agree that the suspicion is there for a reason, either you are very insecure in the relationship (paranoid) or something has changed in his behavior.
I think its a WRONG to place the burden on you and say you HAVE to trust. A persons subconscience works in mysterious ways, it picks up on things that your conscience mind does not, this may be whats going on. I think its important to either prove your suspicions wrong OR prove them right.
I hope your wrong but if your not, understand men cheating is like finding a cockroach in the kitchen. You might find ony one cockroach but you know there are many many more hiding in the walls. You catch him at one affair, of course he is going to tell you this is the first time it happened, it will never happen again, what else is he going to say? Is he really going to admit he has the morals of an alley cat and the women who have been in his bed are so many he can't put a number to it? Of course he isn't. Men confess as little as they have to in order to get themselves out of the argument. Thats all they want, to not argue AND to be able to do what they want.
If he values you so little to lie to you, to use you and cheat on you, this will not change. Walk away now before things get worse and you are tied to him.
My daughters Father had a string of little hotties. I wanted to believe him when he said they were a thing of the past. I started checking on him when I found things didn't make sense. I found all kinds of evidence in almost plain site. I then asked him a few questions to which he gave me lies in return. I let him hang himself really well. Then I told him that I knew he was lying and it was time to confess. I could see the wheels turning in his head, thinking "how much does she know", "how little can I confess and get this over with and still keep her around".
At this point in my life, if your cheating on me, what is there to talk about? Anything YOU (the cheater) say is going to be BS and designed to blame someone else, to make it seem less then it was, to manipulate ME into doing what you want. I don't have that in me anymore. If I am ever involved again and I find out he is cheating, I will wait until he is out of the house, have the locks changed and move his things to the front yard so he can retrieve them when he comes home, no need to beat a dead horse. If your man has no respect for you (which they indeed don't if they are cheating) then hashing it out with them, begging them to be REAL, HONEST will not change anything, other then to make you feel worse then you did in the beginning. BELIEVE me they know they are doing wrong, they know they are hurting you, all the things you tell yourself that you WANT them to hear from you...they know them already, they have probably had another girlfriend yell it at them. Don't waste your time.