Did he tell you all this voluntarily or did you ask him about it?  You need to STOP comparing yourself to this ex.  You need to stop this obsession.  It’s pointless.  Unless you date a virgin, ALL people have a past and ex’s and wasting your time obsessing over them is useless.  

You sound extremely insecure.  I can’t tell if your BF is doing anything aside from following and looking her up.  But if that bothers you, let him know.  

If he’s the one that keeps bringing her up and talking about how great she is, etc. dump him.  

They were together only 4 mos.???  You’ve been with him for 2 years???  And he’s still obsessed and talking about her?

You both sound like you’re obsessed with this girl who most likely doesn’t even think about him.  But for him, she’s the one that got away it seems. 

You need therapy, self-esteem, and to be with someone that isn’t still obsessed with someone they dated for 4 mos.