Spoke too soon?

Oh dear. It was all going so well, but today that run of positivity spectactularly ended.
I turned into one of those people I totally despise and lost my temper with my beautiful, clever, funny little girl.
It wasn't even her fault - she was ridiculously tired after a late night yesterday and has conjunctivitis so is feeling a bit miserable and as a result spent most of the afternoon moaning.
Couple that with the headache from hell for me, no paracetemol in the house and a bout of intense nausea and you have a recipe for disaster.
I knew what was coming, and did attempt to avoid the situation by arranging for backup but Mark was at work and couldn't get away and my parents were shopping. Which is fair enough, on both counts.
It was the first time in months and months that I've felt that I shouldn't be alone with her, and I wish now that I'd tried harder to find an alternative by contacting friends or just getting out of the house.
But there's no point looking at the ifs and shoulds. It won't change anything. I just have to try to learn from the experience and move on.
Reading that back, it all sounds terribly dramatic and actually I'm sure it's something that happens in a lot of houses every day. I didn't batter her over the head with the book I'd just had to read for the 15th time, I didn't strangle her with the cat's tail she'd just been pulling. I just shouted at her.
But I really shouted at her. Proper, grown-up out of control shouting. Not a sustained bout of abuse, just a sentence. But that doesn't excuse it or justify it. There is no justification, in my eyes, for what I did.
I'm not sure I, or she, will ever forget it and the look on her face will certainly stay with me for a while. It's definitely not something I want to repeat. But of course that's obvious.
I'm desperately trying to be a positive, loving, patient parent and of course I shout at her sometimes if she misbehaves (although to be honest, mostly I just use strict voice coupled with a warning about a consequence). But routine discipline, and behaviour shaping, is a world apart from losing control and bellowing at her to be quiet. And she knows that as well as I do.
The whole situation has brought back so many uncomfortable feelings for me that I wonder if actually I was doing as well as I thought.
Maybe I am just not cut out for this at all. There was a moment, after I'd apologised to her, explained I felt poorly and had a headache, and was tired and therefore got grumpy, just like she does, and she refused to give me a hug, that I just felt she would be better off without me.
It sounds ridiculous now but I was close to calling Mark and telling him to come home because I had to leave because I couldn't be near her.
I realise that is a huge overreaction but it feels like such a setback for me that I can't help it.
And of course now many other incidents from the last week or so are coming back with a fresh context, including a conversaton I had with someone who asked me how I was finding motherhood.
When I said it was okay, he was clearly shocked and I knew I had given the wrong answer. The correct one would have been a gushing monologue about feeling fulfilled and enjoying every second.
But I don't think that will ever be true for me, and now I'm wondering if my best, and the compromise we have reached, is actually good enough for anyone.
I'm hoping things look better in the morning.