Trust issues

I have never done this before, but I have no one to talk to right now. I am scared and alone. When I was a child my father was abusive towards my mother, and tried to kill all of us. I don't think about this often, as I usually disconnect from it so much that it seems like someone elses story when I think about it. I have been in a relationship now for 4 years. About a year ago my partner contacted someone they used to have a thing for and were talking casually over social media. I read the messages and my partner never mentioned me and lied to me when I first confronted them before they had me read them. They said they only lied because they didnt want me to think anything happened since nothing did, but since they lied in the first place it has been difficult for me to trust them ever since. I think my trust issues come from my childhood, being let down so often I am skeptical of everone around me and feel like everyone is only looking out for themselves. When I first found out that they messaged this person, I contacted their brother who confirmed with me that they never texted and nothing happened. I want to believe that nothing else happened and move past this, and sometimes my mind drifts away and begins to forget but other times the thoughts come swooping back in. I am in so much pain. I don't know how to stop obsessing over these thoughts that i have. When I try to talk to them about it, they apologize or say they feel bad they ever messaged them but don't talk to me much because they're not good at handling emotions. I'm not sure what to do. I am scared that I'm being paranoid or obsessing too much but how do I stop thinking of these things. Its been almost a year since this happened, when will i stop thinking about it. I feel haunted by these thoughts. I just want to be how normal people are.