Everything.
This is your first time, as you said; it's more or less par for the course. If you look at a lot of the writers that made it decently with TiTs many of them had years of experience in some fashion before starting in.
I'd suggest that you go back, spend maybe half a year to a year writing your own stuff for fun, and do it every day. Get your technical proficiency up to par, then start worrying about your stylistic matter.
Practice, practice, practice.