"Between thought and expression lies a lifetime"
edit: I think most writers dislike thier old writings because they are in a different mindset when they read thier older stuff.
That said, my one piece of advice would be never destroy your old writings. I've had a private livejournal for like 5 years now and I just post ever single thing I write in there (mostly as a failsafe to me wiping my hard-drives and losing it) and even though I don't like most of it, I'm glad it's sitting there being preserved. One thing I will always regret doing is ripping up one of my journals over a girl. That was pretty dumb. I wish I still had it to read, no matter how annoyed I would be with my former self.
The interesting thing is that I'm starting to actually enjoy the OLDEST stuff in there, because I can't really remember the specifics of what I was feeling when I wrote it, or who it is about if it's about a girl.