What do I do?

I am torn between two exceptional ladies. One I've been with for years, the other I've been seeing about a year. The one I've been with for years has been good to me and she's been there through it all. I admit sometimes I am not easy to live with, but she phenomenonly loves me. However, the lady I've been seeing about a year is quite remarkable herself; and she is more fun to be with.
So...how do I tell the first lady I've been with for years, that good ain't good enough?