Sand it smooth with 400 grit wet-and-dry sandpaper… the black stuff.
Spray primer-sealer over the entire panel you are painting. Paint.
Your paint didn’t match because A) Your paint faded (unlikely with white paint) or B) You bought paint you thought would match in a spray can from the hardware or home center store.
If it is B), don’t use that type of paint. It will not hold up on a car and it will not match (as you found out). Buy car paint from someplace like this… https://expresspaint.com/
They also have tips, FAQs and videos to help you.