Struggling with Intrusive Thoughts

Hi everyone!

My partner of 17 years (37M) broke up with me (34F) last April, and I feel like I'm doing quite well now. I'm on meds and I've done CBT, and we've sold our house so just in the final stages of packing up.

Since we broke up, I've upped my hours at work and I got myself my own car, learnt to drive again after 10 years, and just feel like I'm getting on the right track.

When we broke up, I discovered a few things. One is that he started dating an 18yr old a month after, which hit me like a ton of bricks. They're not together now, as far as I'm aware, but it's difficult not to feel like he cared so little for me that he could move on so quickly.

The other thing is that he cheated on me for three years during our relationship. He went onto a dating site, found someone, and drove pretty much an hour and a half either way to be with her whenever we weren't together. We weren't living together at this point so I wasn't aware. This is the thing I'm really struggling with. I can get over everything, and I can get through the breakup and whatnot, but this just keeps coming into my head. How can someone do that to someone they claim to love? Is it really that easy to betray someone?

I had no idea whatsoever, and I thought we were just together and that was that. He seemed like a good guy and I had no reason to think otherwise. It's really screwed me up in the sense of trusting people, which sucks.

I wondered if anyone has been in the same kind of boat and whether you have any tips to get rid of this from my head?

Thanks in advance!