Obsessed with his ex...

I feel like my boyfriend is still obsessed with his ex... We've been seeing each other for 3 years. The first few were rough. Off and on, he was always talking and messing with other girls, I started to do the same, but he'd want me back when we'd break up that'd only last for a week or two. The girl that was the biggest problem was the girl that he had a real first interest in. I don't know if he actually loved her or not, I think he did from what I see. But she always messed around on him which is why he treated me that way because he changed to that person after being hurt. Well in the begging he was still messaging her trying to have sex with her, trying to just see her, take her out(which he never did with me) she'd usually be nice but as far as I know not see him,but she's a huge whore so who knows. When we were getting better and more serious I told him he can't have anything to do with her because it's disrespectful and makes me feel like he wants her more. Well I'd always find he'd still try to add her on Facebook, still search her on Facebook, snapchat her. It went on for a while. But he eventually gave up because all it did was cause more fights with us. And he swore up and down she doesn't mean anything anymore and he wants me. For the past year we've actually stayed together besides when we broke up for a month a while ago because he just wasn't a good boyfriend. When that happens everyone told me they have never seen him like that. He cried everytime his friends came over,he drank heavily, he cried to his parents(which im the only girlfriend that's met his family),it was bad. I'd never even seen him so upset when we've broke up before. We got back together because he changed so much, really got a kick in the ass. I ended up moving back in with him. I have caught him once he looked her up it caused a fight. But we moved on. I figured whatever he's not friends with her he hasn't talked to her, I can't say I haven't looked up exs either. And that was awhile ago. Well tonight he had his Facebook up, I looked through the activity log(just because I'm a snoop) and saw he liked a post from a friend who had that girl tagged in it. I'd didn't see what it was just that it had to do with her. Which is probably why he liked it. I'm really hurt by this because I just feel like he still wants her when I though he was over it. It's hard because I'm like his first real girlfriend&love from what his best friend says. And I'm the only girl he's lived with, the only one that's met his whole family, he wants to start ttc with me. But before and now I'm going to feel like, does he want her but with me because he can't her? In what ways is she better than me? Does he wish I was her? Does he think about her when he's having sex with me/wish it was her? I don't want to think those things but what else am I suppose to feel. And I know if I try talking to him about it he's just going to get pissed and it'll just be a bigarugement😞