All you need to do is to analyze.

What I mean by analyizing is this: Finish a day as you usually do, and when you sleep count the number of productive hours. By productive hours I mean the honest hours you invested in something you really want to do. You sat for 6 hours to study. But you know deep inside that you could have finished that stuff in 3 hours. Then the productive hours would be 3.

Watched a movie? Count the movie length for productive hours. Played football, count the game length after kickoff as productive hours. Spend quality time with family, friends, count it too.

Now when you sleep I guess the count would be like 5–10 hours at max(3–4 hours for me sometimes XD). Now analyze where did the other say 15 hours go(adjust sleep time accordingly). Mostly it would be in Social Media, time pass, or really unproductive stuff :p

Now comes execution part: Use those wasted hours. Try to get maximum number of productive hours. Mine was 20+ in 11th-12th(it involved studies and sleep mainly, but as I said, include movies, sports time, etc. too )

The key to better productivity is concentration and focus. When you watch a movie, watch it with full concentration, when you study, do it well. Avoid multitasking(like listening to music while studying), or using Whatsapp while watching TV series.

You need to have that inner burst which compels you to not waste your day. Don't worry, it will come with time.

All the best!!!