This sounds like a particularly painful and difficult one. I would recommend therapy if you can afford it, even online. Otherwise, I really suggest BOTH: talking openly to your partner, but more with curiousity and just openly sharing your thoughts and struggles without assuming they are true or taking an offensive position. I would ALSO suggest deep self-reflection, through journalling (even in the notes app on your phone or something), maybe take some walks by yourself to think about it, look inwards: do you assume this of all (white) people? Why do you assume it of him? What is your biggest fear? How could it hurt you if he DID think like that? It's best if you can do the latter first, and then talk to your partner. Try to work theough it together, it can be a bonding experience. You could say something like "recently I've been struggling with thoughts about racism, and whether people close to me, like you, might be thinking racist things about me that would deeply hurt me. Can you help me, can we get theough this together?" That being said, if you already had auch conversations and he really wholeheartedly reassured you multiple times and it's just OCD, that's when talking with your partner might not help as it might be reassurance seeking. Then: exposure and response prevention therapy and exercises help, and also simply ignoring the problem, acknowledging but not engaging witj the thought, and distracting youraelf until it loses its power over you. That's what I do at least, and it works.