Write the good story. Feeling your story is good will motivate you to finish it, to work through the problems you encounter, and to get better.
JK Rowling says she rewrote the first Harry Potter five times. That isn't five drafts, it is five rewrites, and then it took a year to sell. I presume she knew it was a good story, and that is what gave her the motivation to grind at it for so long.
Beginning writers (in my opinion) make two mistakes that you are talking about making. First, they think writing shorter stories will make them "professional quality" quicker than writing a long story. I disagree, I think the number of words you have to write to be "pro quality" is about the same no matter what you write. So you might as well be writing a story you love, the novel.
All the short story is going to teach you to write is a short story, and trying to stretch a short story to novel length is very unlikely to work. If you want to write novels, write a first novel. Learn about the three act structure, research typical book lengths for your genre, and put in your practice hours on a novel with characters you love, and a story line you love, and a setting you love.
The second thing they get wrong is thinking that first story they invent and love is THE story. No, when you finish that story, you will have another idea you just love. If you are going to be a novelist, you will likely finish a book every year or two; and every one of them should be a story you love.
A third thing beginners get wrong is thinking their first attempt must be marketable. It doesn't, not at all. Maybe you can sell it, but I encourage you to think like JK Rowling: You may go through a dozen drafts of your first novel before you think of it as "finished." But THEN consider it as a first "Super-Draft". If you wait enough time and read enough about writing and read other novels critically to see how bestselling authors do things, then at some time in the future you can take that same story you loved, and instead of redrafting it, take some notes on the scenes and plot turns you invented, the memorable ones (which means you should still remember them without having to re-read your Super-Draft). Then rewrite it, finding places to use the tricks you learned, so you produce a better "Super-Draft 2".
In short, don't think using a great original story to practice your writing means you will trash a great story. The idea is still 100% yours, if it is publishable then eventually you will publish it. Put it in a drawer, write other stories for practice so you get better, and if you still love the original and want to see it published, rewrite.
Operate on the assumption that having one great story idea proves you are creative and can have many more great story ideas, probably more than you can write. The first one is valuable, but not so precious it must be preserved at all costs. You can have another.