Yeah I did the exact same thing. I'd write a riff or something and a few hours later, come back and change something. Or even just sit there for hours messing with the same riff. I actually did this alot when I first started writting my own tunes. I noticed that I wasted alot of valueable time doing this, not that I'm saying you shouldn't do that. But inspirational moments come and go rather fast. Every musician knows that, just out of the blue you get a really kool idea for a song. Then an hour later your stumped. A good idea to avoid this, is when these inspirational moments come, start planning out how the song with go. With a simple chord progression (or melody), where the tempo should pick up (drum parts), key changes, and stuff like that. Then once you got a nice plan, then go back and mess with the riffs and perfect it. Everyone is different, but I found this has helped me and maybe you guys too. I know everyone has some shape of this problem, and it's not a get-rid-of remedy, but it helps.

Some good ways, I think, to get inspired are listening to your favorite tunes (I'd watch this cause if you listen too much, you end up copying them), maybe reading a book or watching a movie, or even looking at a painting and other forms of art can inspire some ideas.