PPD & Obsessive thinking, insecurities ect.

I was diagnosed with PPD, which I figured I would develop because I have depression and anxiety already. I'm about 2 months postpartum so my hormones are still all out of wack. That said, since about 4 months pregnant I started accusing and thinking my husband was cheating because I'm super insecure. He has never given me reason to think he would cheat on me. It all got triggered when I was on his phone and saw a message from a girl on *** in his dms. He said good morning beautiful and she replied back hello, but the message was never open. His account was hacked assuming by the girl and he swears on his son's life that he didn't message her on there. He knew the girl and found her on an app that they were chatting to each other on, so it's not a completely random girl. He did delete and block her and let me change his *** password. He will let me look at his phone whenever I want and go through it if I please, it has always been like that. Since then though I've just been so insecure and still today think about it and obsess over it. I uninstalled all my social medias because even getting on them or seeing them on my phone triggered my thoughts about all of that. How do I stop thinking like this