The word maySecond attempt (paying attention to title)
Led
Add a single letter, it will make the opposite of me
Fled (Opposite of leading is fleeing)
Add a different letter, it will create my homophone
Lead
Add a different letter, and I won't be so good
Bled (Bleeding typically is not good)
Original attempt
Ad (as in advertisement)
Add a single letter, it will make the opposite of me
Fad (weakest link for sure)
Add a different letter, it will create my homophone
Add (addition)
Add a different letter, and I won't be so good
Bad (not good, obviously I worked backwards here)