Johnathan,
I think that Marty is right, as usual. In the Queen's English, nadir means the lowest point in a negative sense. Following is the Oxford definition:
"1 the lowest or most unsuccessful point in a situation:asking that question was the nadir of my career"
Perhaps what you mean to say is that your current handicap is the nadir of your golf career.
Re your question, I think that for every person there is an innate limit to their ability to improve their golf game. There are many who try for many years with coaching, psych assistance, and practice to become tour players. Most don't make it, no matter how long and hard they try. Becoming scratch is no doubt an easier target, but there are still a small percentage that can achieve that goal. There are many professional athletes, from other sports, who have the physical skills and the mental makeup to be scratch players, but there are few, if any, who can get there, despite trying.