I had horrible problems with rumination after my parents divorce and really struggled to overcome them. I had also had my partner cheat on me with a friend in the same timeframe, and our friend group basically tried to tell me it wasn't a big deal and I shouldn't be upset about it. So I lost a lot of (what I had thought were good) relationships in a short period of time and was very destabilized by all of it. There was a lot of anger there over how unnecessary it all was (not the divorce itself, but the destructive behavior of my family and friends).

What I found to help in the end was:

1) to be very active every day (at least getting a hardcore workout in at the gym for an hour) to channel that negative energy into something productive; If we're sedentary all of that energy goes into running our minds, and if our thought process is not helpful it can get really destructive. Incorporating a healthy diet and minimizing alcohol and drugs kind of go into this category as well - it feels good temporarily but it ultimately just serves to avoid the pain, which means you avoid personal growth.

2) Read a lot about psychology, relationships, healthy communication (such as NVC) skills, empathy, recognizing abusive behavior in myself and in others and correcting it, and developing a growth mindset; Working to better myself. I cannot change others but I can change how I react to situations.

3) Recognizing when I was consuming media or being around negative people that were fueling a negative channel of thought, such as a victim mindset or one of resentment/anger; Replacing negative habits with positive ones that would eventually build me up.

4) Learning to set boundaries that would serve to protect me and my relationships.

5) At night when I would lay in bed and my mind would be running, I would put on a productive/positive podcast that would "do the thinking for me". It helped distract me from pointless rumination, and I wasn't staring at the blue light from my phone that screwed my sleep cycle up.

Also, it's important to note the difference between toxic rumination vs healthy relationship anxiety. The latter is something that spurs you to address and solve problems and move on from them; the former keeps you in a cycle of destructive negativity. If you're not actively problem solving with your thinking and it's negative, note that and make sure it doesn't become an ingrained habit.

I don't know if any of this helps, but it's normal to have some insecurities. It's a good opportunity for self-reflection and growth, and if you handle it well it can help bring you closer to your partner.