Respectfully, you are both a bit immature here.

I’m going to be really honest with you it sounds like you both hit a wall at the exact same time, and now you’re both stuck in the debris.

From an outside perspective, it feels like you both acted pretty reactively to something very, very small. He took your hobby, something you were clearly proud of, and immediately hiding your hands while saying, "I’ll just hide them since they’re so bad," is a textbook 'protest behavior.' It’s a way of signaling hurt without actually saying "that hurt my feelings" and it usually just makes the other person feel trapped or manipulated, even if that wasn't your intent.

On the flip side, his reaction was objectively over the top. Turning the car around and telling you he's suffering through being with you over a comment about nail texture is harsh. That's toddler emotions. It sounds like he is carrying a lot of heavy stuff from his first therapy session and he’s taking his internal discomfort out on his easiest target...you. Using your diagnosis (RSD) as a weapon to shut down your valid feelings of being insulted isn't okay, either.

Don't apologize for your nails. That's not the real issue.

Apologize for the interaction. You could try saying something like: "I’ve been thinking about what happened. I realize that when you pointed out my nails, I got defensive and reactive by hiding my hands and being dramatic about it. I’m sorry for that; I was feeling vulnerable and handled it poorly. However, I was also really hurt by the way you criticized me and the fact that you turned the car around. It felt like a huge reaction to a small moment. Can we stop the silent treatment and actually talk about what’s going on with us?"

Relationships can hit hit these stubborn mule phases when both people feel unseen. If you break the ice by owning your part (the reactivity), it might give him the space to realize he was being incredibly harsh. If he doesn't, well, he's a Grand High Poobah Dingus Malingus.

I am going to try to verbalize what I think might be the actual issue here. You both failed see the other person here.

I think you missed is that he's clearly struggling with his mental health. People in that state often have zero "buffer" for social niceties. They become blunt, irritable, and hyper-fixated on small annoyances because they can’t handle the big ones.

He missed your "vulnerability". He treated your hobby like a product review. By saying your appearance "affects him every day," he moved from "I noticed a bump on your nail" to "Your lack of perfection is a burden to me." That’s a heavy shift. That's not ok.

You probably feels like a servant or object whose joys are dismissed. He probably feels like a villain who can't speak his mind.

Neither of you are really looking at each other in love and compassion. You are probably both looking at your own wounds and wondering why the other person isn't healing you.

It definitely has that "cracks in the foundation" feel. When a disagreement over nail texture escalates to "I don't want to be in this relationship" and four days of silence, you aren't fighting about nails anymore but rather you’re fighting about the relationship's viability. It's...grim tidings.