I’ve been there too - in a way. Terrified to tell a partner about a secret because of the possibility that they may react with rejection or judgment. And the fear is somewhat founded on reality: he could break up with you. It could be horrible, yes. Best to be prepared for that.
However, what you might not be appreciating, and certainly what I did not appreciate it at the time, is the potential upside. If you trust him with this deeply personal and embarrassing truth about your medical status, it has the capacity to bring you together in ways that could not be accomplished by any other means. It might be that he is sitting on his own secrets, and you’re telling him will open the floodgates that will bring you closer together. Absolutely, there is risk in sharing your truth, but there is also a huge amount to gain.
I think the question to ask at this time is, do you trust him yet to hear this truth from you? What things has he done to lead you to trust him? Wet tells you that he is someone who will hold this secret with kindness and empathy and respect? If you’re not at the place where you can trust him, (even acknowledging that you could reasonably trust someone who ends up reacting in unhelpful unfortunate ways,) it might be that spending the night is a little premature at this point. I don’t say that judgmentally, it’s just for me, I would need to trust someone quite a bit before I spent the night.
Anyways my thoughts. I hope you end up finding the courage to tell this to the right person, and that it ends up being a huge benefit to your relationship down the road, even if that ends up not being this particular relationship.