How do I cope with a partner who has relationship OCD?

I recently found out that my partner has relationship OCD (ROCD). ROCD is a form of OCD that gives people intrusive thoughts which make them question their romantic relationships.

My partner feels incredibly guilty about having these thoughts, and only told me recently after they’d been going on for a while. I asked about the details of the thoughts, how they manifested, and got a lot more than I had bargained for—they told me they glorify their old relationship, wonder if they’re really happy with me, and if they are if it will continue, think repeatedly that they must not really love me, and have also thought of me as unattractive. They told me that they constantly have a voice in their head telling them to leave me.

The other thing they told me was that when they are having an “episode” they don’t love me—they said, “everything flips”. I asked if during an episode they’d ever said “I love you” back to me when I said it, and if they had been lying when they said it, and they said yes. That’s really the kicker for me, the lying.

We’ve been together for six months, which feels like a long time given our ages and situation. We’re both 20 and in college, spend pretty much every night together (and a lot of our days, too), and until recently (I thought) we were happily in love. But I feel like I can’t trust them anymore. I know these thoughts aren’t their fault, but they handled stuff really badly by not telling me sooner. I also can’t help but think this is my fault—if I were smarter or more attractive they wouldn’t be feeling this way. And now they’re going through hell, and are completely unable to get these thoughts out of their head. They feel incredibly guilty for what I’m going through with this, and I’m having a hard time hiding how awful it is for me.

TLDR: my partner has intrusive thoughts about leaving me as a result of OCD, even though they say they love me and want to stay with me. They tell me about these thoughts and I feel like shit. I don’t know what to do.

How do I support them without being totally miserable myself? Has anyone been through this?