OMFG.
First off, **** that guy, seriously. All he has done is worked on you to try and make you want he wanted and then the real thing changed it's mind and he is dabbling with both of you.
I don't mean to sound like a bitch, but grow a spine girl! You should have NEVER accepted this in the slightest, you should have dumped him the first time you found this information out and disappeared out of his life with the bat of an eyelash.
NO one should ever be someone's second choice, and if I were you I would leave this relationship, take the time to recover emotionally and properly set my prospects, and then find a guy who IS WORTH YOUR TIME! : ( I hate reading posts like this. In the end it is just sad and everything is always easier said than done.
Get angry and tell them both to **** off and just leave. If you've been giving him your best why does she get the best of him? Not right and this just means it's time to pack up and go.
Good luck to you. You seem like a really nice lady, but you don't need to be trampled on. Toughen up and let us know how it goes!
“I was never really insane except upon occasions when my heart was touched.” ― Edgar Allan Poe
Wish for a pile of shit to turn into gold hard enough and guess what? It's still a heaping pile of shit.