Sorry to hear that.
It's funny when you think you know someone and you don't even know them at all.
I think that if he is able to carry on like this online, he would easily be able to carry on like this in real life.
Even though I'm sure you don't want too you have to wipe away the tears and you have to confront him.
It might hurt like hell, but you've busted him betraying you.
If you chose to stay with him, and work things out then you might be only setting yourself back up for heartache.
Even though you can verbally say that you're not going to take it.
It doesn't always work that way with your heart.
An easy way out just my suggestion...
You make a profile on a site he goes to msg him, and compliment him.
Do what you have to get him to talk with you and get him interested. Then make a date.
Tell him you want to see him, and if you could get together. Be prepared because I'm sure you know he might jump all over this especially if you just paste a hot pick of some girl. When he sets the date you print everything he wrote.
Then throw it in his face, and then you will be able to say.
I caught you I know for a fact. It's more than flirting and more than some sort internet world for you. You've been unfaithful, and the trust is gone. ect... you're over
That's only my suggestion.
I've been there before, and I've had to go through the exact same thing. I was foolish I forgave but in the end I just kept getting hurt and they kept lying. Nothing changes. You saw his true colors and he is going to show someone else.
Just thank God that you know now instead of before you're married because he would of took vows before god and then when he would betray you it will hurt even more.
I hope you get through this
Best of Luck