Jealousy and trust issues*

I ended my first relationship of 9 months a few weeks ago and reflecting back on it I noticed I am really disappointed with myself at how jealous I was a few times, mainly me getting a nagging feeling whenever she would go out that she would kiss/fuck someone else. There was no reason for this as she was a very honest and kind person, but this feeling still persisted.

Perhaps it's because we went on a break about 7 months in and after two weeks she had sex with 2 other guys, whilst I did not feel ready to do that even though I had the opportunity. This really fucked with me and makes me pretty anxious that the person I'm dating is going to cheat on me, even when there is no reason to. Shamefully I have to admit that this sometimes even leads to me checking when she was last online to guage until how late she is up.

Does anyone know a book to help me with this? I've been recommended no more mr. nice guy, but I don't feel like I'm a nice guy (when a girl tells me she just wants to be friends but I want more, I move on). But perhaps there are other lessons I can learn from that.