Why am I like this ☹️

I’ve noticed I spend all day every day talking myself out of freaking out that my fiancé is doing something bad. Literally all day everyday. Like I get anxiety on my 25 min drive home from work every day cause I begin to think “what if I get there and someone is there” or things like that. When I get home, I search for things that would lead to someone being there. He doesn’t know I do that, but I do. I have urges to go through his phone and sometimes I will and then put it down and an hour later I’ll feel like I need to view something again or go deeper. When his phone goes off I ask him who it is always. If he asks me what time I’m gonna be home I purposely don’t answer Bc in my head, someone was over and he wanted to make them leave before I got home. We’ve been together 6 years. 3 were in high school and the last 3 we’ve been grown adults, engaged and living together. In high school he was a huge flirt and he wasn’t very faithful until we became adults and made promises to each other. I’m not sure if I am still holding on from highchool or what but I just feel so crazy. Like this is just my life 24/7 In my head yet no body knows why I’m down and upset all the time. It’s cause my brain thinks I’m needing to solve something and I’m just tired. He hasn’t done anything in 3 years to make me think he’d cheat or go behind my back. How do I get over things from highschool? Why am I like this? I need help.