Back from the dead

Wow. Not bad, as hiatuses go.
But I'm back now, you can all breath easy again.
No, seriously, I never meant to take such a long break, but things have been hectic, and I'm a lazy cow at heart and... and... oh, you know how it is...
Anyway, quick catch-up:
Great holiday in Italy (not shacked up with either George or Brad, unfortunately, but very close, at least geographically), OK summer, C and L changed schools, my father came for 3 weeks (as did D's mother, at the same time of course), I've been working and well, that's it, really.
Ah yes. The girls.
Oh, people. I'm in a dark place right now. The change of school has gone fine, essentially. For C, I thought it would be harder than it was and I'm really proud of how she's coped with the change from a class of 8 to a class of 25. For L, it was tougher, with lots of wailing in the mornings. But she's fine when I go to collect her in the afternoon, and she's made friends, and her teacher says all is going well.
BUT.
L is being sooooo difficult at home - she won't help out, she won't do anything she's asked to do, she has tantrums... I'm at the end of my proverbial tether (any further off the end and I'd fall off, in fact) and spend much time either screeching like a Harpy (and making my beautiful girls cry) or crying myself (which also makes them cry). I feel like shit, I feel like such a failure as it's obvious that L is desperately SOMETHING (unhappy, disturbed, insane, stroppy... I just don't know) and I don't know what to do.
This week hasn't helped, with D away in England since Tuesday and not due back till tomorrow. I have a new-found respect for single parents - this gig is TOUGH!
I'm exhausted, desperate and prone to long bouts of sobbing.
I love my little girls, I love them more than anything on earth. But I'm failing them, I'm not making them happy, I'm screwing up their childhood and I'm terrified they're going to grow up hating me, never remembering the good times we've had together (yes, unlikely as it seems, there ARE good times).
I feel wretched and terribly, horribly alone - the friends I want to see the most are in another country and I just don't have the same friendships here.
Tears are streaming down my face, my sweet little L was sobbing when I left her, all I want to do is run to school and scoop her into my arms. But when she IS here, she bugs me to death at the moment, doing everything she can to press all my buttons. It's the ultimate lose-lose situation. I ache to see my girls when I can't, and yearn for them to be elsewhere when they're here. Not all the time, not all the time, I swear. But this week. And lots of times all the same.
I don't deserve them, my angels. They're sweet and kind and bright and beautiful, and I love them, really, really love them.
But I'm no good. No good for them.
The only thing I'm fit for is work.
I often feel like I should just walk out and never come back. They'd probably be happier without me. But I don't do it, and will never do it.
What's gone wrong? Why is my life such a fuck-up? Why are my girls so difficult?
Help me, someone, please.