I don't think it is understandable. First of all, if you want to end a friendship, at least tell them. Second, why not tell her why you are ending it? She will no longer be your friend so the fact that you think it is not understandable, that doesn't really matter.

I also don't understand why her being successful at something and you being not is such a big issue. If you want to be successful, better surround yourself with successful people.