Aww, I know how you must feel. *hugs* Ive had stuff go wrong in the lab, recently we've been having a load of cell culture trouble, affecting everyones work and not just mine. Its been getting quite a few people down.
I'd try not to worry about it too much, they dont expect perfection from a Masters project. In fact, Im sure a lot of first year PhD students have similar problems. Its just a matter of finding your feet, its just that Masters students dont have the luxury of time to endlessly repeat things.
If youre getting to the point where its seriously depressing you and affecting your work then I would personally have a word with a supervisor or welfare tutor.