I hate being stuck in my own head

My anxiety and depression is taking over my life. Going through a rough patch in my relationship right now and we're on the brink of breaking up, not because of my mental illness but I must admit it doesn't exactly help the issues we're having. I'm constantly over thinking that I'm not good enough for him and hes going to find someone prettier, and happier than me. I used to be so bubbly and outgoing and I keep thinking he'll find that in someone else. I've also put on weight after being together for years and having a baby, what if he finds someone skinnier? I worry all the time that he's talking to someone else or flirting. Deep down I don't think he is but even then I'm not 100% I hate being in my own head and I can't stop over thinking. I wish I could be normal again. I try and be positive around him because I don't want to depress him but my head is like a tornado inside