Hi, this is my first post to the forum. I've been in a similar boat as you, with a project that got started in high school that's been knocking around in my head for too long.
Here's my advice for rewriting.
*Stay true to the original vision of the story while re-writing. There was a brief phase in the process of rewriting my comic where I completely changed the premise but kept the same general plot points and characters, It was an interesting experiment and it helped me figure some things out, but ultimately it was a failed experiment and I went back to my original premise with a fresh set of eyes and newfound appreciation.
*Make small changes and carefully think through how they affect the story on the whole. I had a lot of scenes that just weren't working and the pacing and tone would feel wrong to me, the only thing that needed to change was something simple,like a line of dialogue here or there. The other benefit of making small changes is that they can easily be undone.
*Don't be afraid to rearrange the order of some scenes. Let's say your plot looks like this:
1) The protagonist sets out on their adventure.
2) The protagonist defeats a great evil.
3) The protagonist has an important character moment as a result of accomplishing their goals.
Maybe something about this doesn't work, the pacing feels wrong or the character motivation doesn't make sense. What happens if we rearrange the plot? 3, 1, 2 make it so the character change RESULTS in the adventure happening. 2, 3, 1 makes it so the character goes out for more adventure after accomplishing a major goal. These are different stories, but they include (mostly) the same scenes.
*Change the roles of your characters. I had to do this because the ending I had didn't feel like it was earned by my protagonist. The protagonist and their love-interest split up. In an early draft this came out of nowhere, and was due to the actions of the significant-other character, and it felt unfair to have that end happen to the protagonist. I changed the roles to make the protagonist less sympathetic and had the split-up result from their actions instead, and this ending resulted in a much more satisfying conclusion.
*Set a deadline or an accomplishable goal for yourself. I didn't do this and It made it so my story got trapped in revision hell for 4 years. There's always something to improve or change, you will never create a perfect story because it doesn't exist. Say to yourself "I'm going to give myself X months to revise this story" or "I'm going to revise this story until Y part of it makes sense." and once that deadline is met or that goal is accomplished move on and just start making the thing. Most people will enjoy it despite its flaws, or they won't even notice those flaws. Once the project is finished (or abandoned) you can take what you learned from it and put it into a new project.
