I often wonder why i have such excellent ideas for Fanfiction's but when it comes to writing them i have trouble?

I'm reading a Fanfiction by an author called Purple Emperor. She's got some great idea's going on one of my fave stories (Discworld, and, it's about one of my two fave characters, Havelock Vetinari ^_^)
I'm unfortunatly still waiting for the end of the story (C'mon Purple Emperor, I can't wait any longer!) but it does give me even more ideas for stories i could write. I sit infront of the Laptop and stare at the screen thinking...
"Ok... how do i start this damb thing?"

Of course, then there's the time when i think of an excellent start to a story but then spend the next six months trying to finish it. God damb creativity is hard! Ah, to be an author... well, to be a GOOD author. lol