How to cope with a stressful job?

Is anybody a teacher / in a very stressful and busy job? I'm 12 weeks 2 days and I'm finding it so hard to juggle all my marking and planning on top of my morning sickness (which I mainly get in the evening!) and fatigue - I get home from work and just want to sleep, not start marking exercise books :( I am also a head of year so I have extra responsibilities on top of normal teaching stuff. I LOVE my job but I feel like I can't do it properly at the moment due to being so tired all the time! My colleagues are now aware that I'm pregnant, which helps, but I don't know how long I can carry on working myself to the bone without something giving. Has anybody got any advice?