Death of a spouse or breakup of a marriage or long-term relationship can trigger similar responses in a person. Each person mourns a loss differently. However, there are 5 common stages of grief a person goes through when mourning the loss of a relationship.
You may not experience these stages in one fluid order. You may go through some of the stages more than once. Sometimes an event can trigger you to experience one of these stages again. For instance, cleaning out the basement and finding an old shirt of your deceased spouse or hearing your ex-partner is to remarry might cause reoccurrence of certain stages. The five stages of grief are:

1.Denial - The "No, not me" stage.

This stage is filled with disbelief and denial. If your partner has died you still expect him to walk through the door. If your partner has asked for a break-up you think that she will change her mind.

2.Anger/Resentment - The "Why me?" stage.

Anger at the situation, your partner and others are common. You are angry with the other person for causing the situation and for causing you pain. You might feel anger at your deceased partner for dying. You may feel anger at your partner for asking for a divorce and breaking up the family.

3. Bargaining - The "If I do this, you'll do that" stage.

You try to negotiate to change the situation. If you've lost a spouse to death you might bargain with God, "I'll be a better person if you'd just bring him back". You might approach your partner who is asking for the break-up and say "If you'll stay I'll change".

4. Depression- The "Its really happened" stage.

You realize the situation isn't going to change. The death or break-up happened and there is nothing to bring the other person back. Acknowledgement of the situation often bring depression. This could be a quiet, withdrawn time as you soak in the situation.

5. Acceptance - The "This is what happened" stage.

Though you haven't forgotten what happened you are able to begin to move forward.

Adapted from Elizabeth Kubler-Ross, 'On Death and Dying'