tryingtolive,
It seems very common, and it's certainly something I've had to deal with.
I think looking back at the past starts as a way to try to understand things. "I'm screwed up, how did I get that way?" Look to the past and find the answers. Then trying to understand the past makes one look at the past. And a habit forms. Eventually one is always staring back at the past in shock as though it can change. PSTD is a super helpful aid in this department

!
Fortunately, the only power the past has over us is power we give it by looking at it. What we give, we can take away.
You can change the habit, and it doesn't control you. You just have to change the habits of mind that push you to keep looking over your shoulder.
The only way I know how to do that is with practice. When the urge comes over you to look at the past, recognize it: say something like, "I see you arising, urge to look at the past," then ask yourself if you want to give in. "Do I want to listen to the voice and look at the past, or just stay in the present where most things are safer?"
Sometimes the habit will win. Then I think it's helpful to recognize that I chose to give in. Today I decided to look at the past. Maybe tomorrow I'll make a different decision.
Sometimes the urge feels like its out of your control, but it's only a part of you that wants and needs that look. That part is an addict, and it needs your help. It's addicted to the flood of cortisol (or whatever other chemical is being produced), and it wants that fix. Start restricting the flow. Gradually ween yourself off the drug of self-created depression by choosing consciously to look away. I find it's helpful to have a specific safe present tense thing to look at that you enjoy and that implies forward motion. Then you start having a habitual go to place to replace the past with. Then reward yourself when you make the healthy choice. Notice that, too, and you'll see over time a new habit starting to form.
I did a lot of research on will power for stuff like this (it takes a lot to keep it up!). One helpful way to build will power: deliberately change your posture. If you cross your legs when sitting, for example, start putting them both down flat. Research shows that these little will power games help the will power muscle in the brain.
Best wishes,
Danny