Try not to feel guilty - what you describe sounds pretty much how I was when ds2 was born and I didn't have four children and I had my dh around. I was quite horrible to ds1 at times, felt I wasn't coping, his behaviour was "challenging" and I remember thinking "what have I done?" Just try and get throught this one step at a time - don't plan too far ahead (carpets are lovely but you will manage without for a while). You know you have done the right thing, your kids and you are better off without your ex-p - you WILL get through this.
Make the most of your time tonight with just the baby - try and relax, veg out in front of a video or have a bath and an early night. When your children come back tomorrow sit them down and say you are sorry you were so cross, but you are very tired and things are difficult at the moment - stress you still love them and always will, but you can't help being cross sometimes. Praise anything good that they do to help to the skies!
I am in awe of your bravery and determination to make this new life for you and your children. I hope things start to come together soon.
Oh and I second the Homestart idea - once you are in your house it would be an excellent idea. My friend used them when she had two under 3's - and she said, after a while, it was just like having a friend in the house to help out (the Homestart volunteer would mind the children while she went shopping, or mind one so she could have time with the other, or mind them both just so she could have a bath in peace!)