Hang in there FF. I've had similar problems with some of my tutors too.
Some are so picky that they nail every grammatical error (sometimes ones which even aren't actually too! - eg they would criticise me here for writing aren't instead of 'are not'). I've just been pulled up for not following proper footnoting conventions - hell, I've been doing them the same way for the past 6 years - as outlined in my student handbook - so I can't figure out what was supposed to be wrong with them!
Times like that - it gets me so fired up that I determine to do better next time where I can - other than that, I've done my best and I can do no more.
Maybe that's what your tutor is trying to do with you - maybe he knows you can do even better so he's pushing you - only thing is, it doesn't work for everyone, it can be a demotivator as in your case at the moment.