First of all let me tell you, there is no such thing as “Main hardworking period” in life. If you want to be successful, you have to follow a consistent hard work!

No one becomes successful in a one night fight… so you have to be patient enough to see the results of yourself being a hardworking person throughout the life!
For some people, there are periods of life when you were more hard working while some other periods when you were lazy. Everybody has their own ups and downs in their lives… But the most important thing in all the situations is to be patient and hard working.

I wish I had started earlier. But, you know what? After almost on the verge of completing two decades on this earth, my new favorite place to visit would be my bed. I’m usually tired by the end of the day because I've been working harder than I did before. Yet, I've never felt more happy and accomplished. Which makes me wonder what could have been had I started sooner. 
Nevertheless, I didn't. Why? Simply put, I didn't know then what I know now.

So here are some tips for you to be a hard working person:-

Start early!
yes that’s the most important thing… start your day early in the morning. by waking early, you can plan your day according to the priorities. and planning the day and sticking to the plan..is very important!

Failing Isn't as Bad as You Think.
It could have purely been due human error – like forgetting to carry the one in a math problem. That just means you’re human. Plus, you’ll learn from your mistakes. I mean, if you messed up that one test item which cost you, say 10 points, wouldn’t you be more diligent the next time?

{As Thomas Alva Edison said, “I have not failed. I've just found 10 thousand ways that won't work.”}

Practice positive self-talk.
Introspection is very important. Use present tense messages when practicing self talk to remove future worry with positive assertions. Talk yourself through fears by asking yourself what the source may be and how you intend on correcting it.

Time Is NOT a Commodity.
You are young and have many things ahead of you. Choose to use your time wisely because time is a privilege NOT a commodity. Time will let you find who you want to become and create who you choose to be. It will bring you the experience of success and failure.

You cannot change your past, nor can you know what's inside the mystery box of your future. All that you have in hand is the present you. so, start being hardworking from now…

hope it helped you!

Work hard, Be patient and Amazing things will happen for sure…

Thanks for reading my answer (^_^)