It's impossible to reduce an ability.
About the only thing you can do is reduce its exercise
or the willingness to exercise it.
-L. Ron Hubbard from the lecture "Control"
An obvious example is an athlete who gets injured. He still has the ability but he can no longer exercise it.
Next is reduction of the willingness to exercise the ability. Let's look at an actor like Silvester Stallone. Nominated for two Oscars and a WGA award in 1977 and a BAFTA award in 1978. Clearly the guy has the ability. Then he picked some not-so-good movies to appear in and then the press and the creeps at the "Razzie" awards started to target him and next thing you know he is a "star" because of his past record and not because of his current films. Recently, probably because he was out of the spotlight, the attacks decreased and he made a bit of a comeback, gaining some critical acknowledgment for the movie "Rocky Balboa", which he wrote, directed and starred in.
So, how can you apply this to your life? Where are you not exercising an ability as much as you should? Is it caused by an unwillingness to exercise the ability? Is it caused by something else? Injury, illness, drug abuse? Narrow it down by examining the ability and what may be stopping you from exercising it. Once you've figured it out then you can figure out possible solutions that will help you to exercise it more.
Tell me how it goes.