Most Helpful Guy
I am going to assume that this new relationship has been in the works for some time.
She must be eager to snatch him up and she's whispering into his ear all the things he wants to hear. You should have noticed a change in his behavior around you.
But now that we are at this point I can only advise that you make peace with this unfortunate outcome and move forward. Hopefully after the fallout of this broken relationship there will be something left saved; an amiable re-friending.
I wish I had been able to speak to you 5-6 years ago, before you moved in together. I have stated this before and I would have urged you to not to move in with a guy unless you're already engaged to be married.
While a broken heart takes time to heal, at your young age all is not lost. You have time to begin many new relationships. Take what lessons you have learned from this and get back in the game. Choose wisely and don't move in with a guy until there's a ring on your finger!