I can't stop obsessing about a girl my boyfriend dated recently...advice please!?

My boyfriend and I met a year ago, and we've had a pretty rocky relationship dating off and on until here recently. For the past 5 or 6 months he's become comfortable enough to call me his steady girlfriend, and we are making plans to move in to a house together. But...here's my problem. Last Thanksgiving we had a huge fight, and he met a girl while I was out of town visiting family. We had decided to work it out, but then I got a text telling me he'd met someone and he still wanted to be friends with me, but he wanted me to know he was talking to someone else. Well, it didn't go that well and they ended up breaking up after like a week and a half due to some drama taking place in her life, and according to his family she freaked out on him and cut off communication. His sister told me that he was infatuated with this girl from the moment he met her. He brought her home to meet them almost immediately, and said he felt like he'd met the perfect girl for him. She was "an angel", his "soulmate" etc. etc. And of course I checked his Facebook during this time and pretty much every single post on a day to day basis was about this girl. So after she quit talking to him he turned to me, and I stupidly took him back. I know I probably shouldn't have, but just like he felt a strong connection to this girl, I felt (feel) just as strong of a connection to him. So we started talking again, and around March he moved into my mom's with me. So we've been fine since then, haven't really fought a lot, seem to be closer than we were, and I thought everything was fine. One day I came home from work and he had left his Facebook open on the computer, and out of curiosity I went to the search box and typed in the first letter of her name "C". Ok, so tell me why her name is the first name that popped up in the search box?? He had her blocked not too long ago, and now she's not only unblocked, but he's searching for her?? When I asked him about it, he wouldn't admit it, but he wouldn't deny it either. He just said I was "way more important" and he loved me. So now I'm obsessing over it, and it's starting to affect my moods and I'm irritated and mad at him all the time. Now I check the computer every single day and look at the temporary internet files and cookies to see if he's searched her name. She actually knows who I am and she blocked me from Facebook after her and him broke up. I'm friends with everyone in his family, and we kind of know people in the same circle - she actually sent me a message once calling me a "ho" because I was snooping her page from my touch phone and accidentally poked her. (I know...dumb). Anyway I can't see her Facebook from my account now so I have to log into my cousin's Facebook and check her page. And I do this every single day now! I can't stop! It's compulsive, and it's gotten to the point where I feel like a stalker. Let's face it...I guess I am a stalker. My best friend even said I was becoming a stalker because I started googling her and trying to find out more information about her. But I was fine when I thought she was out of the picture and now it's like there's a possibility that she could come back or something and it scares me to death. Is it me? Is it him? Is it her? Does he want her? Do they want each other? How do I deal with this? If I could get an objective opinion or perhaps some insight from someone who might have gone through a similar situation it would help me so much. Thanks guys...