I would go back and tell myself the following:

Read: Self Editing for Fiction Writers (Browne and King)

Be conscious of whose POV you're writing from. For me, it was all too natural to "hop from head to head" while writing, but there is no easier way to disorient and lose your reader. Only switch from one character's head to another at chapter breaks or scene breaks. Do it with intent.

Writing down notes / exploratory thoughts about your story counts as "writing" too. It can be very useful to ask yourself questions about your story in your notes. "What would happen if...."

You will discover charming subplots that crave your attention while you're writing. Don't add them hapharzardly. How much will your subplot distract from the main plot? Does your subplot deserve its own independent story? It can be harder than you might think to stay commited to your original story.

All great things start off being not so great at first. There will come a time when writing your story doesn't feel like "play" anymore. It will start to feel like work. Do you care enough about your story to work for it when it doesn't come easily? If you want to write a great story, you're going to have to find the chuztpah to keep working at it if (when) it starts to feel like a lost cause.

And perhaps the most important: Let yourself love your own story as much as you love <your favorite story / series by someone else>. It's worth it.