Guys, Really need a guy's perspective preferably who has gone through something similar. Should I stop overreacting?

pinkmuffin
My bf and I have been dating for almost 8 months now and I won't deny that I am the overly jealous, emotional and anxious type.
Cutting to the chase, troughout the first few months of our relationship he has been talking A LOT about his so called "crazy" ex gf. Almost every thing would remind him of her. after a while, he would talk about her more and complain about the things that somehow irritates him because "it looks like what she used to do".
Saying all this, I am very sure that he is actually over her since he has always been very pissed everytime he talks about her.
we are both musicians and he writes songs. he has a particular song that he considers his favorite, or "a chart topper" if it was to be released one day, and it is about her. Though he he has made it clear that it does not affect him emotionally when he plays that song anymore as he treats it as nothing more than just one of his great works of art.
My problem with this is although I have convinced myself that I have gotten over the whole emotional idea of the song, whenever he plays that song on his spare time it would still get to me.
Basically, the reason why I'm confident about us is that I never hear any good trait about her, or the relationship. she was a very disturbed person and would ALWAYS look for him to vent all her anger onto him. while he gave her a lot of chances to try and change, she won't and that was why he walked out.
to add, I was snooping around his phone today and got very curious about them so I read their msgs to each other from 5 years ago and got pretty upset at how he was begging her to reach him as the convo was about how he wanted for them to work out and how he loved her.
So with the basic picture, I just can't help being constantly upset. I really need a guy's perspective about how I should take this. What could be on his mind and do you think that I should be worried that he regret leaving her? what should I do to stop feeling upset about "nothing"? thanks!
Guys, Really need a guy's perspective preferably who has gone through something similar. Should I stop overreacting?
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