The time that I experienced the most fright was my grade 12. (Or you can say... Grade 12 was the most frightful/fearful period I have ever experienced) In my country, every student in grade 12 has to take an entrance exam to get into their favourite university. However, to make it happen, (In order for that to happen), I had to study very hard and also under (immense? maybe you can use the word to describe the intensity of the pressure) pressure. Every morning I woke up at 5 o'clock to review my lesson, then went to school at 6am until 12pm. After spending all of the morning at school I just had (spending the whole/entire morning... I had only) 1 hour to have lunch and take a nap. Extra classes from 1pm to 9pm really made me sick (were you disgusted by it or were you feeling tired?), sometimes I didn't even have time to have snack between next classes. Returning home with lots of homework waiting for me that made me go to bed at 2am everyday, I was really tired and afraid of the next day. (Usually, I returned home with lots of homework and every night, I had to stay up till 2am just to finish it! ...) I didn't have time for my self; too much pressure, homework, paperworks and I also didn't know which university would suit me. All of those things made me frightened whenever I think of them. I will never forget the time in my grade 12 (my time in grade 12) which frightened me a lot. (it was truly frightening)

So instead of lumping too many ideas into a single extremely complicated sentence, you can break them up into shorter simpler sentences. It makes your ideas and sentences smoother. Complicated sentences often confuse the reader and writer so, keep it short and sweet!