I know how you feel.
I was with a bloke for 4 or 5 years, we broke up, then got back together again for another 6 months or so before breaking up again. I had suspected a few times that he'd cheated on me, but no proof. The last time we broke up, he rang me 2 days later to tell me that he'd been out for dinner with someone, but it just wasn't the same not having me sit across the table. I mean, WHAT THE?! TWO DAYS?!!!!!
Anyway, they were engaged within 3 months, and the whole time he denied that he was seeing her when we were still together. Stupid man forgets that I am also very good friends with his housemate, who told me everything.
But it was a very hard pill to swallow and there were times after both breakups that I did not cope very well at all - some of the scariest and worst moments of my life.
But the revolving door keeps moving, and I've since met and married my beautiful husband and we're looking forward to many many years together. And it's true - you do tend to meet someone when you're not looking and not expecting to.
Hope you're feeling better about it soon.