I would say that my first success was finishing my first song...I say this because it took me a hell of a long time...I started playing keyboards at age 9 and drums aged 12 and guitar then as well...I always tried to write my own stuff from the very beginning...For some reason I could never finish a song...I would get a riff or some chords together but not know what to do with them...sometimes I would get half way through a song and just get stuck and leave it and start something new...I was at least into my mid 20's before I ever finished anything...
Now in the last 10 years I've finished dozens of songs...some instrumentals and a few vocal tracks now...I'm just so glad I never gave up trying...!!!
I guess any future success would be performing live or making a living from music...In a way though I have something better than that...I've posted a few of my tracks in the listening post and had a really good response from members and instructors...for me that's more important than making money...!!!
:D