@Ken,
Pick up a copy of Stephen King's
On Writing, if you haven't.
I started writing stories as soon as I could draw letters. Wrote a small novel in fourth grade. At 18 I decided to make a concerted effort to write short stories every day to improve my writing and develop my own style.
I hadn't really written much fiction in years when I read through
On Writing at about age 32 or 33. I'd mostly just been writing on forums and Girls Chase, adding in non-fiction writing styles. I wrote one novel at age 29 but other than that had done little fiction for quite a while.
After I read through
On Writing I came away with a wealth of great ideas and new things to try. Quickly belted out two new novels. Later I went back to working on my scifi saga I cooked up around age 19 and the writing I did for that outclassed anything I've done before.
On Writing played a big role in helping me to improve a lot there.
There are also some really excellent writing/character development videos on YouTube that are worth watching. There was a channel I got a lot from a few years back (which I can't now remember, unfortunately). It introduced me to character squares, where you take the four most important characters in the story and place them on a square -- each character mainly interacts with the other two adjacent to him, with his values/goals aligning with some and opposing with others -- as well as to character arcs, which I'd never paid attention to. Got me to writing entire backstories for characters and creating character arcs I wanted to focus on.
Also useful to think about distinctive character traits: flaws, shortcomings, skewed perspectives they have on the world, and so on. Makes your characters a lot more fleshed out, and not just carbon copies of the same one character reskinned to look different or have a slightly different backstory or purpose. The different characters' different ways of viewing the world tend to lead them into a lot of natural conflicts as you write, and the interesting thing about most writing is the conflict (within the character, with other characters, with the environment, etc.).
Chase