Help!

So I have been talking to this guy online now for about 4 months. I have started to fall for him. We were even talking about visiting eachother (we live in different states). Everything was going great, we started coming up with nick names for eachother and just getting along so well. Than he went MIA for a couple of day. So I got curious and looked up his name on facebook..That's when I saw it....he has been dating a girl for 3 years...I instantly couldn't breathe....I had an anxiety attack and ended up passing out for 30 minutes...My sister came home and found me in the bathroom and ended up waking me up...I'm crushed...And I don't know how to bring it up to him...Help. I don't know what to do...