Why am I obsessive over this?

[Grilleall]

I appreciate good answers & no judgment against my question. Thanks in advance. I started dating my bf a year ago & we had facebook etc, a certain girl would try to be all buddy buddy with my bf & it infuriated me. She would even be snarky & that's when me and my bf decided to delete facebook. She would hang around some of his friends (of course he was always with me & never hung out with those friends) and we never hung around them and drama happened now I still obsess over what she's possibly doing. I put it in my brain that my bf likes her and she's way better than me. It's taking a toll on me. My bf and I had serious issues because of me and this problem. I feel like she's prettier, cooler or whatever even though my bf says she's gross and not someone he wants to be around at all & even her personality is depressing. I have been told plenty of times how much my bf loves me or even likes me in comparison to her. and yet I find myself looking up her social media.... Why am I doing this