If he thought you were cheating.. why wasn't his first reaction to question you and have it out? When I thought my partner was cheating I didn't jump in to bed with someone else! I think this is a load of rubbish personally and he's trying to get himself off the hook.
It's not going to be easy but I really think the best thing here is to let go

it wasn't just the one indiscretion.. he's admitted to cheating with more than one person on more than one occasion and I highly doubt he'll stay faithful in the future.
xxx