Sometimes they call me "Sunny" because I'm a very soft person and try to stay positive in every situation.

You need an article in front of a noun.

But I also know that my friends and family have seen my depressived side and they have supported me on my way to becoming a human again.

Also I was a straight-A student at school.

However, at the Uuniversity my grades dropped and my knowledge remained solid only in interesting areas of science.

You wouldn't capitalize "university" unless it's after a specific name, such as "Cornell University" or "Shanghai University". If you're talking about university in general, it's common to simply say "at university" or "in college".

To me, this is a really difficult question to me.

You generally specific the subject (ex: yourself) at the beginning of a sentence.