How do I become a great musician?

I've wanted to be a musician since I was little. I've been singing for as long as I could remember and I've been playing the piano since I was 14. A year and a half ago, I realized that I wanted to be a composer. I also want to be a songwriter as well. I've written many lyrics and I think I'm getting to the point where I want to write the music to them. How do I keep up with all that by pushing myself everyday? I want to be excellent. I also want to play the guitar, harp, oboe, vibraphone, xylophone, clarinet, and drums. Oh, cello and violin! I need tips on how to keep up with singing, songwriting, composing, and playing the piano.