I think it's great that you apologized. I mean, so many times I feel if my wife would just say "Sorry, I'm just not feeling well", it would not bother me so much. But when she doesn't, I let things get to me, and they start eating me up inside.
But having said that, I'm afraid it wouldn't work. Why? Because I wouldn't trust her. I hate to use the dog analogy, but it just seems so fitting. I mean, if you keep kicking a dog, and then one time say "Oh I'm sorry, I'm not feeling well", and you pet the dog, do you think the dog will immediately come to you? Probably not.
Also, when you write it, it totally makes sense, like you say "another person comes out for two hours." But I think I have a much easier time understanding you, and every other cyclothymic in this board. Why? Because there is no way it affects me, there is no way in my mind I can make it my fault that your sister-in-law was looking around in FB. Yet, with my wife, I can always make it my fault.
I sometimes wonder whether my low self esteem is the result of my wife's cyclothymia, or maybe it was always there, but now I've just become more aware of it. Regardless, nobody said it was going to be easy.
So like I've said about taking care of yourself first, before taking care of a loved one. Truer than ever.