My boyfriend and I been dating for 6 months. Friends for 4 years. 

He was in a relationship with a friend of mine for 2 years, she is extremely toxic. I can give examples if you want.

After the breakup she started slandering him on facebook, instagram and twitter. This worried my boyfriend but I know her too and I think she is just looking for attention.And I think no reason to worry because she doesn’t mention his information 

Before we started dating he would often talk about how obsessed he was/is with her. That he would never love anyone else or stop being obsessed with her.

We started dating 10 months after their breakup. It was ok but he kept mentioning things about her, I got fed up and told him to stop. He promised to never mention her again. 

Today he \[accidentally\] mentioned his ex and instantly backpedaled. I asked what happened and he "confessed" that he checked her again but that it's not what I'm thinking and he only wanted to know if she's still lying about him. I don't know if I should believe it. 

I am not a logical person all the time. I work on myself and follow what I learned in therapy but, I’m not cured and I have moments where BPD wins. That’s why I came here to ask for another perspective. Thanks