My last relationship I ended lasted six months, and I told them in person that I didn't feel that she was the one I wanted to marry, and spend the rest of my life with, and that I didn't want to waste either of our time.
I told her that I did like her, but I didn't think she was the one. Although I think it hurt her, she appreciated my honesty. Today, we are both happily married (just to other people!).