Should I stay in this relationship?

[supernova30]

My boyfriend and I have been dating for the last 1 year. In the beginning of our relationship he told me that there is a girl in his life, who's 10 years younger than him. She is into him but he's not in love with her or physically involved with her. But that since she loved him, he didn't want to break her heart and she would move on soon enough and he wanted to be with me. I thought it was a juvenile crush and didn't pay much attention.

Yesterday the girl wrote to me on facebook that she was his girlfriend for the last 2.5 years and that he was cheating on her. She had found out through stalking my facebook, twitter and tumblr profiles. I confronted my boyfriend who begun yelling at me and accusing me of being the cause of this situation where this "poor girl's" heart was broken.

Now I'm in two minds if this is a relationship I should be in. He doesn't care about my feelings or me. He is supposed to protect me. Instead he and the girl have victimised me. Please help.