If you were able to send a letter to you atr 15 years -old-self, what diwould you say? If you were able to send a letter to your 15 year-old-self, what would you say?

When you're talking about yourself you say: 15-year-old-self. You can of course also for example say: her 15-year-old-self if you're talking about someone else

In my case, the advice iswould be "Gget enough sleep well", because I realized I had been in lack ofing sleep at thate time. In my case, the advice would be "get enough sleep", because I realized I had been lacking sleep at the time.

When saying "sleep well", you only mean how well someone has slept, you don't mean how much. Therefore you say: get *enough* sleep.

I had to study, join a club activity, and play video games, so I often cut on my sleep time. I had to study, join a club, and play video games, so I often cut on my sleep time.

I'm sure what you mean about "club activity, you would never use that in English.