If I’m in a relationship but not engaged, and I find that boyfriend/girlfriend has character traits that clash with mine, what is the best way to end the relationship?

In this case, you are called to treat your loved one according to the golden rule: “As ye would that men should do to you, do ye also to them likewise” (Luke 6:31). You would not like someone to tell you, “You are a terrible person! How could have I been so deceived to trust a person like you? You are a loser! You’ve wasted so many days of my life!” Don’t ever think like that. Focus on the good aspects of the relationship you are terminating. Be humble and be gentle with the heart of the other person. You may say: “It has been a privilege to know such a special person like you. I respect you for so many reasons! But I understand that I’m not the right person to make you happy. I see that we are not compatible. Thanks for all the beautiful moments with which you’ve gifted me. I wish for you to be happily married one day to a person better than myself. I will remember you as a great friend.” In other words, don’t make the other person feel guilty about the failure of the relationship. Be nice and gentle with his/her heart. Ending a relationship is extremely painful. Why add more to its bitterness?