One of the most important lessons I have learned in life was when I did my school project. I began to make the presentation to this the day before the defense. It was half past one when I have finished. I pressed the save button, but, suddenly, my computer stopped working. I managed to fix it only by rebooting. The presentation has been deleted. I felt myself very awful. Luckily, on the next day my mum agreed to help me. Together we made the presentation in one hour and I passed my project to the highest score. Afterwards, I have understood, that time is precious and that we shouldn't put off till tomorrow, what we can do today.
У тебя с временами нормально. У тебя тут почти ошибок нет.

разве не Learnt 3 форма этого глагола?
граммотнее будет - in MY life
presentation to this the day (to this я бы убрала)
I pressed the save button, but, suddenly, my computer stopped working( можно переделать с использованием времени, которое тут у тебя ни разу не встречалось, но это так, как вариант. у тебя тоже граммотно. - Suddenly, my computer stopped working when I was pressing the save button)
on the next day ( оn лишнее)

с временами у тебя всё в порядке, не наговаривай))) а  вот предлоги не очень любишь.