Tough one... Why ask, pray tell? Either way, if you have to convince yourself that you don't love them, then you probably do love them and should pursue it. But if that is impossible, for whatever reason, it is better to move on and know that they were a good person that was worthy of your love and affection rather than destroy all good thoughts and memories of them.
In the event that it was a bad relationship, and you are absolutely convinced that you have to forget them, focus on all the bad aspects about them. Everything they did wrong or that irritated you.