He cheated on me with her but I didnt know!?

If they've been together for 2 years, then technically he cheated on her. Hate to be the bad guy here because I know you're crushed! But look at it this way, this Josh guy's a total creep! He has a girlfriend, yet he's messing around with you. Not to mention, you aren't even of legal age and he's 20. Does he really love you? No. Does he want to get married? No. A real man wouldn't do this kind of thing to even one girl, let alone two. If I were you, I wouldn't have anything more to do with this jerk. No need to break up, because it was never a real relationship in the first place, he made sure of that. And I would steer clear of online romances, they are seldom worth the time you put into them. It's true a lot of guys two-time girls through the internet. They feel like it isn't really cheating if they don't have sex with the girl and that makes it okay. But it isn't, people like you get hurt! They're only doing this to stroke their own ego, and love themselves more than anyone else. Save yourself! Best wishes.