I don't know what to believe?

My boyfriend's ex girlfriend who admitted to still having feelings for him and has caused problems for us in the past has told me multiple times that he cheated on me with her. He has looked me in the eyes on several occasions and has calmly promised me that it's not true. Based on her past of lying to get the outcome she wants (such as lying about being pregnant when my boyfriend broke up with her for the final time), I don't find this hard to believe. However, she has resurfaced and is still saying the same thing. She's blatantly and obviously lied about things regarding my boyfriend before, so I know she isn't to be trusted, and I know when my boyfriend is lying, and I've never recognized his denials as lies, so I really want to believe him. I just wanted to get some other people's opinions on this. It's driving me insane. She won't go away, and I don't know why!!! She pretended to be my friend at the beginning of my current relationship with her ex too!