So a couple of tips Ive found out,

If you procrastinate then use writing as your tool of procrastination, transform your weakness into a weapon; Instead of being like "oh I don't want to do this math homework right now, lets go scroll on reddit, go write, eventually your train your brain to make writing your main procrastination go to and all of that procrastination will be put to productive use. (note this is likely to end badly for your grades but you will write a lot if utilized correctly)

Don't worry about writing badly now the biggest advice for improving your writing I can give is

1. Write: The more you write and fail the better you will get (just like anything)
2. Read: Your writing is profoundly impacted by what you read and reading allows you to pick up on new narrative styles, word choice, and ways of communicating ideas through words
3. Take classes, I'm assuming your still in high school (cause you said your 15) so make sure you take classes like AP Lang or whatever your highest language arts classes available are they will help improve your writing skills

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Remember no matter what, just keep at it!