Wrirting Challaenge, June 4th

I believe when I was atin high school and preparing myself for a university entrance exam, I wasted a lot of time daydreaming or lazying around the house.

I wasn't a bad student at all, but I didn't try as hard as I could, and I regret it terribly even after many years.

Now, that I, as an adult, know how precious time is, I try to use it more efficiently.

Wrirting Challaenge, June 4th

I believe when I was at high school and preparing myself for athe university entrance exam, I wasted a lot of time daydreaming or lazying around the house.

I googled and read that Iran has one university entrance exam called Konkour. If you only prepared for one exam, we would use the. If each university has their own exam, you can use "a". It just depends on the situation. Edit: sometimes we like to use possessive pronouns (my, his, her, our, their, your...) to avoid using articles (my entrance exam/her entrance exams)

I wasn't a bad student at all, but I didn't try as hard as I could have (tried), and I regret it terribly even after years.

From Perfect English Grammar: Could have + past participle means that something was possible in the past, or you had the ability to do something in the past, but that you didn't do it. (You can repeat the verb in the past participle, but if you don't use it, it's fine). This grammar is a little bit more advanced, so please don't feel a lot of pressure right now :)

Now, that I, as an adult, know how precious time is, I try to use it more efficiently.

The comma after "now" isn't necessary.