Do you have as much work to do as a substitute? I suppose as its a long term position you will do.
Don't get down about it I'm sure your a great teacher, I was talking to my little one's teacher yesterday (it came up because she asked if my boyfriend was a teacher... he wanted to come to parents evening but had a meeting so missed it) and all teachers feel the same, at least the ones we've seen. Unfortunately there's so much extra work and red tape ect. that it often gets in the way of the actual teaching and you don't get to see the good parts as much as you'd like. Try and relax with it (yeah easier said then done I know) and not let the extra work get on top of you. I say this to my boyfriend all the time, but he never listens. I usually have to shout at him for getting all stressy with me for it to work... do you want me to come and shout at you?
I have to agree with everyone else as well, 15 syns is fine its within plan (I haven't done 15 syns a day yet but think I might actually give it a go one week just to see how I get on) and could you try and eat more as well? as you don't seem to be eating much :/