Things are going wonderfully with my boyfriend. A week or so back he left his Facebook open on my laptop and he kept saying how it's because of this certain conversation he had with an ex girlfriend that is part of the why we are together. So out of curiosity expecting that I already knew the jist of the conversation I opened it and found out that he couldn't "make up his mind" between me and another girl and that he'd sexually been with a lot more girls than he claimed he had to me. (double the amount)
When I brought the subject up again he still told me the same thing he had told me before. And I believe him because I love him. But doesn't that mean he still has a thing for his ex? Still trying to impress her? Or is he just lying to me trying to seem like a better person even though I told him I wouldn't judge?

I know the only way out of this is to tell him I've read the conversation but I don't want to loose him...
Please help :(

... And that's what you wind up with when you lack trust and start snooping...

Yep, got to agree with odinn7...
You can't win now. You are suffering from indecision, lack of trust, and guilt all wrapped up in one little snoop.
What you have to realize about trust is that it requires a LOT of time to develop, and that you can never go by what you hear or read elsewhere or even in a private communication that doesn't include you. People LIE, and that's all of us, even you. Double the number of sex partners? Making up his mind? If you want to take that at face value, that's your downfall. Your first sentence was about how wonderful life with him is.
Your choice how you learn from this and what you do next.
Now that the deed is done and you are all distraught, you might as well confess, apologize, and deal with the consequences. But if you choose to try to deflect your snoop with accusations, you are bound to lose him.

Oh, the tangled webs we weave...

He may be lying on Facebook, assuming you are teens most likely, most lie to many others about all the sex they have. But of course he did not want to seem like a person who was just out there sleeping all over, so he lied to you. It was wrong, but then honestly that question should have never come up, he should have never discussed it with you.
His past sex life is just that past.

And he is with you ? Correct ? Then does not seem like he has not made his mind up

You're right - the only way out is to tell him you read his Facebook.

You already don't trust him. After you tell him that you read his FB he will no longer trust you.

Doesn't sound like a stable relationship or one with a future to me.

He may be lying on facebook,

He can't lie on the internet. I know because I read it on the internet and nobody lies. Right ?