A bad habit! (trying to write in the past)

I had a bad habit when I studied in my first year at university; I didn't study hard enough for my exams.

That habit affected me because I didn't pass all my subjects and I felt really dissapointed with myself. 

I always thought that studying only a few hours per week it was enough and I would preferred to do another things instead of studying for my (university) classes.

But then I thought about my bad habit and began to study hard during the next year. I put my head down and felt how my bad habit was changing. I will never make the same mistake again. 

All this taught me, that I always need to try my best, in order to be a great student.

bad habit.! (trying to write in past )

I had a bad habit when I studied while studying my first year at the university because I didn't study hard enough for my exams. That habit affected me because I [didn't][wasn't able to] pass all my subjects and I felt was really dissapointed with myself. I always I used to think that if I studied only a few hours per week, it was would be enough and I then I could do other things beside would prefer to do another thing except studying for my university classes. But then I reconsidered my bad habbit and get down to focused on studying hard for my next year at the university. I worked hard and tried to put my head on. and I felt up to change my bad habit and so that I would never do make the same mistake again.
All that teached this taught me that I need to try to always be do a great student always.

Good job!

bad habit.! (trying to write using past tense in past ) (I won't fully correct but add to Heinz's correction or offer alternatives)

...

always I always used to think that studying ied only a few hours per week it was enough and I would prefer to do another thing except anything but study my university subjects classes. But then I reconsidered (reconsider is fine) my bad habit and get got down to studying hard for my next year ...

to do anything but... = do anything else excepting ...