Your sentence is understandable and acceptable but not as natural as it could be.
I would say (as Jazz also did) that using "being" in your sentence would make it better.
--> "I went from being a top seller to the top-performing salesperson"
I would also say that replacing the word "terrible" with "under-performing" would make it more professional too.
--> "I went from being an under-performing seller to a the top-performing salesperson"