I did the same thing with Anders, I forgot to give him something and I had to go back an entire act to redo it to get the ending I wanted. On the plus side of replaying, I discovered a whole slew of quests and interesting things (also changed some of my responses) that I missed the first time around. It didn't take me that long to redo it (but then I play a lot), so redoing an act wasn't as bad as I thought it would be. Just my two cents.