I hope you don't mind my asking, but do you want help with dealing with guilt and sadness, or to explore doubt about the decision? Possibly wrongly, I think I sense the latter.
If you don't want to go there, please ignore the rest of this post; for what it's worth, I think the decision as it stands is a completely legitimate one.
But, it seems to me that there is more than one thing going on here and maybe these need teasing out and looking at separately.
You say you've agreed as a couple that two is enough yet, for you, three is something that feels right on some level. Deep down, would you like three?
Although I've never had significant MS, never mind HG, I hear how much you've suffered with it. However, were you to wake up tomorrow and find the sickness and nausea gone, how would you feel then? Would the decision instantly change? Not likely to happen, but would it tell you anything about whether the decision is the right one?
I feel for you - it sounds like the worst kind of head and heart decision.
(Tbh, I'm torn about posting this because I don't want to add to the problem - forgive me if it's unhelpful.)