Can you help me decipherer my strange dream?

Really cool dream. You have to break it down to the main points and focus on the emotions you felt during the dream rather than the actual subject matter of the dream. From my perspective I'd say Betrayal and Fear are 2 of the main points. You feel alone and afraid and you have a lack of trust for people. For some reason or another these feelings are tied to your old school, perhaps you lost friends when you left that school. But I think it's more likely that its because you see the time you spent at that school as a simple and stress-free time in your life when you felt much more secure because you weren't worried about what your going to do with the rest of your life, and you feel that those times have slipped away. You fear that you might not succeed in regards to your career choice. That's why that guy threw you off the roof. But your not ready to give up and that's why you bit his toe off. The mangled cat is another fear/disappointment thing that represents your fears of life not turning out the way you want it to. The cat goes back to its mother in that cozy safe place just like you want to go back to being a child when you weren't scared of what lies ahead. Try to believe in yourself and stay positive, I'm sure everything will turn out wonderfully for you. And know that its healthy to have these feelings.