Writing is Difficult

Do you find writing difficult, or does it just kind of come to you? If you are the latter, I envy you.

When I started writing, I guess it was the end of my grade eight year, I could think of anything. I started this story, and I never really had a name but I called it For Keeps. My closest friend read it, and fell in love with it. I guess that's when I really started.

Now, I have started so many stories it's crazy. I can barely ever finish them because I lose interest. Do you ever lose interest? If so, how do you keep going? Where do you get inspiration to write more?

I was thinking about starting a short story about something supernatural which is typically what I write. What do you like better:
a- Greek mythology (demi gods and such)
b- werewolves (a-oooooo!)
c- vampires (nom nom, bloood.)

Or something else? Please comment with your answers! Thanks.