once a cheater??

Shannon
I recently found out my boyfriend of 3 years had cheated on his a past girlfriend (the one before me). He told me himself when it got brought up in random conversation. They were extremely dysfunctional and fought constantly, and not that him and I are perfect but our relationship is a lot different from what I've heard from him, mutual friends and the ex herself. Anyways I have never thought that he would cheat on me but now I can't help but wonder if it's just in him forever and it just hasn't happened yet! We live together, family vacays out of the country, have plans and talk about the future, etc. and don't plan on breaking up anytime soon. I'm trying not to be insecure or crazy but I'm a girl and of course think of the worst sometimes 🙄 Has anyone else dealt with this type of situation? Am I being ridiculous? 
Sincerely, 
The overthinker