Your writing is quite good. It is probably better than that of many American high school students.
I would only suggest a few minor changes:
- decisive is spelled wrong
- 'decide about something': 'decide something' or 'make a decision about something' are more natural
- 'study': 'studying' or 'studies' are more natural
- 'university entrance exam' is more natural
- 'every time' is two words
- 'waste some hours': this is OK, but 'waste several hours' or 'waste a few hours' are more natural
- 'anxious all the more': this is OK, but 'even more anxious' is more natural
- 'without any plans': this is OK, but 'without a plan' is more natural
- 'is really uneasy': this is wrong. It should be changed to something like 'makes me really uneasy'
Great job!!