My bf's estranged wife wants him back?

...and he left our apartment to go live with her instead. I am sad, upset, and i feel like all this time i just wasted it on him, and he was just ready to leave me for HER. And he said he was divorced the whole time! Why do guys do this? How do I get over it? I just wanna forget about him, but i can't. Why did he live with me for so long (4 months), if he was just waiting for her the whole time?