Cecelia's having a hard time

The last couple of days have been amongst the hardest I've seen as a mother. Cecelia has been restless and fussy and sad and I can't figure out for the life of me what is causing it. She's not sleeping regularly at night (or for naps) and she's upset all day.
I have really been struggling with her lately, and I am running out of ideas and ways to keep myself sane. The curse of always having had such a wonderful baby, is that I don't really know how to be a mother to a normal baby. I feel so overwhelmed and stressed with the task at hand, and it is scaring me, the mom I seem like I'm becoming. My house is going untouched, because Cecelia needs my every moment of attention, and this is causing me even more stress and frustration.
I just don't know how to describe the feeling that has come over me. Maybe it's impossible to understand until you've been a mother, I don't know. I feel angry and then guilty, frustrated and then sad. All she has to do is smile again and I can very nearly forget all the bad. But then all she needs to do is spend an afternoon crying and it seems impossible to remember all the good.
This afternoon was particularly hard. I had plans for the day and Cecelia was not going to let me carry them out. She screamed while I was leaving the grocery store and at the bank machine, then she fell asleep on the way home. I thought I was getting some relief, but she woke as soon as we got in the door and was up and fussy and squirmy for nearly an hour.  Finally I didn't know what else to do and so I called my husband. Sometimes I find it even harder once I get on the phone with him. No one seems to understand the feelings I'm feeling and I don't seem to ever be able to hear that one piece of advice that will keep me sane and get me through.
Today I had to do what I think every mother dreads doing. I had to leave her to cry. I was so nearly mad, I had to leave her by herself for a few moments and catch my breath. I used those few moments to do a speed cleaning of my house, and was really impressed with all I was able to accomplish. I was feeling good until I got back to her and heard her screaming and coughing and choking on her tears. How can they have such a hold on us? And how do you ever learn to get past it? I cried myself for a few moments and then I went in and I picked her up. She instantly stopped crying and snuggled in to me, as if I had saved her from a beast. It's almost as if she had no idea that I was the beast in the first place. How do you get past that heart wrenching feel of having abandoned your baby? It was only ten minutes and she was fine at the end of it, but how do I convince my heart of that, even when my brain knows it's true?

 All of these things I never ever thought I'd have to deal with as a mother. All of these things that I thought would never plague me, that would never get to me, and would never bother me. I am bothered by them all, and not sure what to do about it.
I am not asking these questions rhetorically to the internet, if anyone has any actual ideas, I would so much appreciate some advice and some guidance. I just don't know where to go from here, and I'm worried about what's coming next.