Try to write more! Do it creatively so it's something that you enjoy. Have your friends read what you write and provide feedback. Writing helps you formalize your ideas and allows you to see errors you make concretely.
Recently I've started reading modern scifi/mystery novels and found the vocabulary to be very simple (smooth? Like the text is lacking texture) by comparison.
If you can't get through novels without falling asleep, try audio books (preferably unabridged). Lots of classics on https://librivox.org