Thanks so much for all the responses. It's good to know that I'm not the only one who feels likes this, as someone above said you feel depressed that you're not meeting your own expectations of how you should be as a mother. And then I find because I'm feeling guilty about failing the kids, and my husband, and myself, then I feel worse and so I get snappier or sadder and it all goes around in a circle.
Sleep and tiredness are definitely a factor, I probably get 5.5 - 6.5 hours sleep on average, always broken as I'm a light sleeper even at the best of times. I used to get maybe 7.5 - 8 hours before Christmas but I've gotten worse lately for staying up later, or going to bed and then staying on my phone well past midnight so that is definitely a habit I need to break.
I do also think PMS doesn't help things. I would say I feel sad or generally just low most of the time, but then coming up to my time of the month I just can't handle things at all, and the small things that I usually ignore - lack of help around the house or just kids being kids - just build up and I explode over what should be relatively minor things. Probably because I do bottle it up the rest of the time so it festers and then when hormones are added in I just can't manage it. Has anyone found anything that helps with PMS? H is just gone away on another work trip, he'll be back next week so I will make an appointment for the GP then, but in the meantime maybe there are supplements or something I could try.
Regarding time to myself to be fair I probably am my own worst enemy there. If I said to H right I'm going to the gym every Tuesday from now on and you need to be here, or said I'm going out once a month or whatever with friends he wouldn't give out (much!). So I'll start researching things on locally in the evenings that I could go to, and maybe toddler activities that I can take DD2 to in the mornings while DD1 is in school. I've been reluctant I suppose to do something just for me as I felt it would be selfish, but the way things are right now I really think I need time for myself away from the house, and the family, so that I have more energy and patience when I am here - even if that does make me an awful mother to need to "escape" from them.
H and I aren't great either for making time for us as a couple, in the evenings we tend to sit on seperate couches and each on our own phone, or watching tv but rarely go out together - probably three times in the last 18 months without the kids. We haven't any family locally to help but I must ask parents in the school if they know of any reliable babysitters.
So that's a nice list for me of things I need to do to get my ass in gear and try to get out from under this and maybe feel better about myself again. Thanks again for all the advice and supportive messages 