Regret about unfinished business is one of the most painful aspects of grief.
Remember that relationships are imperfect, and you did the best you could with what you knew at the time. Focus on what you did give - your love, presence, and care - rather than what you didn't. Consider writing a letter to your loved one expressing what you wish you had said. Some people find comfort in speaking aloud at the gravesite or creating a ritual to honor unfinished conversations. Forgive yourself for being human.