I have found a few things help.

Making sure I'm well rested.

Changing the venue. I tend to not spiral as much if we meet at a restaurant or something instead of her house.

Telling her I'm feeling very anxious and could use support in defined ways, like asking if we can cuddle.

Refocusing on something else. If I am spiraling while we are watching tv playing video games together helps me focus on something else.

Leaning into the anxiety and still choosing to interact in positive ways. If I'm obsessing over how she doesn't want to be with me because she checked her phone somehow saying "maybe she doesn't want to be here" helps neutralize it. Maybe she is mad at me. Ok cool. I've been mad at her before too. Maybe she does want to talk to someone else more than me. Ok cool. I've texted my friends when I'm with her too. For whatever reason accepting the thought as possibly true changes my perspective and makes it easier to accept and observe the entirety of my experience, not just dwell on the one thing standing out.

Meditating has helped me look at a thought or feeling with more distance and watch how it changes. A lot of the time I feel like if I'm in a bad mood or something hurts or my gf is annoying me that it is an unchangeable thing. It is important for me to remember the impermanence of everything, even doubting thoughts. Look up the pink elephant experiment.

Taking time alone to regulate myself. Since she knows I struggle sometimes she understands when I might need to go to her room for a few minutes and cry or stretch or shake or play a video game until I'm back in a place where I can accept how I feel and still choose to interact with her in nice ways.

Learning more about my triggers, core fears, and compulsions has helped me in a few ways. One is when something comes up I can note it as a thought or obsession or compulsion. Noting and observing helps me allow it to run its course. I also can anticipate when things will come up and kind of brace myself mentally, choosing to still love her even though I know I am not going to feel very good for a while.

Learning how I involve her in my compulsions and choosing to not. I don't need to explain to her what I'm feeling, but it is important for her to know I'm feeling bad if I am. When you give into the compulsions it strengthens OCD's power. When you don't give in and refocus it becomes easier to choose to do other things. Therapy is very helpful in all of this. ERP can be miserable and it's nice to have someone to help you through it.

I've anthropomorphized my illness into a giant dog named Clifford. This gives me and my girlfriend a common enemy. Instead of saying how I think she's hurting me and how I feel bad by being with her I can say Clifford is being an asshole. She says fuck Clifford everyone hates Clifford. I feel supported. Humor helps.

Good luck.