I'm also a single mum and work full time and my DD's 4. As she gets older I'm finding it easier; when she was 2 I was fine and then for almost the whole year that she was 3 I frequently used to cry from sheer exhaustion. It does seem to be less draining now (although I still have my moments!).
The things I do to keep relatively sane are;
Waking up at the same time each day; it is a killer waking up at 6.15 on a weekend morning but I find that if I try to sleep in a) DD wakes me up which I never seem able to cope with well and b) I feel worse if I do get an extra half hour/hour's kip
Once I'm awake I get up; a wise Mumsnetter said that once and I recently started obeying it and do feel better, having a bit of quiet time to have a cup of tea/potter round the kitchen/have a shower in peace can make a big difference.
If I feel tired during the day when I'm with my DD I've learnt not to sit and rest, I get out and about as much as possible (DD won't let me rest at home unless I've taken her out and worn her out first!). The more exercise your DC gets the better - swimming, park, anything like that and as much fresh air as possible. When you get home have a picnic lunch/tea, get some toys out, put the telly or a DVD on, and slob out. I sometimes doze off for a little while after we've had a busy morning and wake up to find my DD either dozing next to me or quite happily drawing/watching telly.
Taking a day off a month to myself. Take your DC to nursery/childminder as normal, go back home, put your feet up and relax, or do the little jobs that are bugging you that you don't always have time to do. I've learnt from experience that if I don't have this time to myself I go completely mad.
I work flexibly and end up doing an hour or two's work most evenings once my DD's in bed but as an upside I do half a day from home one day a week then have the afternoon off with my DD, and not commuting that one day seems to make a difference. Is there any chance you could do the same?
Final thing is be good to yourself. Some days really are crap from beginning to end and the only thing to do is get your DC into bed as early as possible, eat a quick tea, and go to bed stupidly early.
I hope this helps 