My partner has OCD and the only thing that helps is accepting the thoughts, whatever they're about. If you try and get to the root of the thoughts, or if you're responding to the thoughts with anything other than a mental shrug, you will be strengthening the loop of thought -> compulsion -> more thoughts.
The compulsive aspect of ROCD can take many forms, including following the thought (e.g. "oh god, if we break up I'll have to find somewhere to live by myself"), checking your physical response to the thought (e.g. do I feel anxious when I think about breaking up? If I don't feel anxious that's a bad sign), asking for reassurance, and so on. I know it's really difficult to accept the thoughts (and to sit with the anxiety), and it may not feel like it's working, but it will help decrease your anxiety over time.
One thing that has helped for my partner and me is learning how to talk about OCD without always talking about the thoughts. So he might tell me that his OCD is bad today, but he won't say "I'm having thoughts that you might hate me", or whatever it is that he's having thoughts about. This might be something you and your partner want to try - it's much less stressful for me, as I know generally how he's feeling but not any upsetting details.
Remember: your thoughts are just thoughts, they do not always reflect reality. Everyone has thoughts that are intrusive or upsetting, but people who don't have OCD just shrug these thoughts off most of the time, or think "huh, that was a weird thought". If you have OCD it is like having a fire alarm that goes off every time you put some bread in the toaster; the alarm is there to keep you safe but is far too sensitive and is being a nuisance, rather than reflecting actual risk to you. Once you know this it's far easier to ignore the alarm!
There are some really good resources online - you can find a lot of helpful things on Google. However, be careful with this - Googling ROCD/your feelings can be a compulsion, and can feed the anxiety.
You might want to try accessing professional support for OCD, but if you do make sure that you are seeing someone who is an expert in OCD, as bad therapy can be much worse than just self-help.
I hope you are able to find some support that works for you. Take care 