Great advice already here but I'll add my two cents on the subject of magnum opuses and projects that are dear to you.
I had a story that I felt would be my biggest and best story ever, with 4 intertwining books and 4 additional spin-off books. So 2 quadrilogies about a fantasy world with mythical creatures galore. I wrote the bulk of it in college. Did a bunch of sketches and character designs. But I never got around to drawing pages for it.
Now that I'm several years removed since its conception, I realize that there is still much to be worked on. The writing was sloppy, events seemed to just come one right after another. Problems in the story were solved a touch too conveniently I feel, etc.
So now I don't see any of my stories as my magnum opus, my one story to beat them all. I feel it's better to have a bunch of good work than be a one-hit wonder. Produce enough series, let the audience come to the conclusion which is best.
Don't feel like you're tossing one of your stories to the beginner mistakes dogs. See it as an opportunity to learn. You might find whatever story you pick to do can grow to be even closer to you since it's with you for your first baby-steps into comicking.
As for myself, my first foray into comics is a slice-of-life, so it's fairly non-committal, but I do have an over-arching story for it. I jumped into it because I was inspired by the art I was seeing at the time and thought, hey, I can do this too. Then my second is my jab at the romance genre, but mostly an excuse to draw nice-looking men. My third was even tighter, it started from a rejected comic anthology submission, and I thought the idea worked well enough that I could publish it myself. All my stories are pretty different, so I suppose that is one aspect that drives me to actually see them to fruition if not for my own benefit.