How can I ever trust a man after being cheated on?

I was with my ex for two years, we had moved in with each other and planned on getting engaged soon, and the when I was out of town visiting my sister, he broke up with me and before I was even back in town he was In a new relationship. My friends moved me out while I was away. He claims he never cheated, but I found out he had cheated the entire relationship. It's been about 5 months and I'm getting back into dating but I'm terrified.