Bf has a huge thing against his ex...

Before I went out with my boyfriend, he was dating a girl I knew that treated him horribly. She wouldn’t give him the time of day, but would get upset if he didn’t towards her. She had multiple guys she would talk to on the side, and would tell him she’s choosing between him and another guy. After a year on and off, he broke up with her after he realized how horrible she was. Sometimes he’ll randomly talk to me about it and tell me she ruined his life. He was in high school at the time and played football. Since she was doing all this to him it made him not focus as much on football, making him not as good over the year. He’s never forgiven what she’s done (I don’t blame him), but today I found something in his notes.

It said “*her name*, I hate what you did to me.” I saw it and didn’t say anything. I don’t even know how to approach this. Like I said, he talks to me about it from time to time so I know about it, I just thought it was strange. He always tells me though that he wish he had met me earlier so he never had to go through that. He’s told me that I helped him get past that and am the best thing ever. I just find it slightly annoying that she’s on his mind quite often. My logic is that if you dislike her that much, drop it. She fucked you up but move on from it and grow. I don’t see why he keeps going back to that? How do I even approach this???

P.S. we’ve been together for almost 4 years now, so it’s been a while.

UPDATE: I ended up asking him about it and he told me he was just really angry that day and had to write it out. I told him about therapy but he’s dismissed it...I think I’m his therapy when he does talk to me lol. Thanks for all the help!