Today is a sad day, how should i?

me and my guy have been together a year. we met off personals in the internet. a while back i put my ad back up cause i was bored and wanted to see what kind of attention my pic would get, and he discovered it and it was a huge blowout thing, but i took it down and told him it was a stupid mistake. he said he should put a profile up to then, and then i asked him a few days later if he did and he said no im not going throuhgh all that internet stuff again. well last night we were on his computer, and where you type in addresses, a few different dating sites came up and he tried to click off them when i pointed it out and asked what was up. he finally admitted to it, and said he did it after he saw mine. either way he lied to me, how can i believe he hasnt been on all the time and what should i do? ive told him i need time to think. need some advice from others point of view. i am really hurt and shocked by this. i feel like i dont know him anymore.