I'm not depressed and I'm not a teacher but I really look forward to going to bed. it's the best bit of the day. I sing in a quartet and we practise in the evening. One of the others sometimes notes how long before she'll be in bed when she arrives at the start of the rehearsal. It doesn't sound as if you're enjoying your job too much so it's natural you should focus on something nice. If you don't want to go part time could you move schools? I know several people who've felt like you and who have left teaching altogether, with nojob to go to, and they're managed one way and another. But it's not easy and I can quite see why you wouldn't want to give up the security. Maybe you can start exploring what else you could do, while you're still at your school.