Grieving and mourning are both related to the death of a person or a pet. Grieving is usually more serious and occurs closely after the death. "Grief counseling" is a therapy service for people coping with the death of a loved one. Grieving is normal, but grieving can be a stressful and traumatic experience. People going through grief may stop taking care of themselves, not sleep, not eat enough. They may require the care of a professional like a therapist. Mourning is usually over a longer period of time after a death. Usually, a person in mourning is back in the routine of daily life, but they may be still in low spirits. Regretting is sadness over something that you have done. It is a feeling of "I wish I hadn't done that" or "I wish I could go backwards in time and make a different decision."

Grieving and mourning are almost always to do with being very sad about someone's death. Regretting is just a general feeling of wishing things were different. Regrets can be serious or less serious, for example, "I regret not bringing a jumper because it is cold." Sometimes 'grief' and 'mourning' can be used to describe things other than literal death - "The was mourning all over Brazil when they exited the World Cup" - but they are used to describe much more serious situations than 'regret'.