You practice! Just like with other forms of art, writing requires a lot of practice to build confidence and skills!
There are many different ways that you can practice! You could keep a journal and write out ideas that you have. You can take prompts you like and build off of them! You could do some exercises where you just describe things, like feelings or seasons or colors! Or you could take up the pages by getting to know your characters better! I myself have so many pages talking about character plots and backstories because they’re always getting tweaked or changed.
And if you don’t feel confident enough in your own characters yet I would really encourage writing fanfiction! Seriously! I discovered a lot of my own style of writing when I was writing fanfiction. I figured out how to make characters that I loved and how to write in ways that made me happy.
You can also take notes when reading books that you love! You can take notes in your writing journal or right on the page (as long as you own the book, of course). Pick out sentences that hit your heart or metaphors and similes that made what the writer was trying to convey very clear! Make notes about their characters and why you like them or dislike them! Dog ear passages that give you inspiration!
I would also like you to try and have a growth mindset when you write. I know it can be hard to write something and reread it only to hate what you’ve written. I know that feeling. But remember that your first draft is never going to be perfect. Every single person on this planet is constantly learning and improving on their abilities. Instead of thinking “I should quit” think “I can improve!”
I wish you so much luck! I know you can reach your goals! <3