The first outcome I came to was grief. But, after more deliberation I have finally concluded that the emotion that has the most effect on us is: Regret. Think about it. Regret is a side-kick to any emotion you have.
Grief: You loose a loved one, and you feel regret for not spending as much time with them as you would like to.
Heartbreak: You feel regret for ever letting yourself get hurt by that person.
Anger: You feel regret for whatever the anger led you to do, or regret for feeling anger in the first place.
Regret can be as simple as "I really shouldn't have worn these shoes tonight." to "F*** I wish I didn't get so angry with a gun in my hand."
Regret is the one emotion that just eats and eats and EATS at you. There's usually NOTHING you can do to change the reason you feel regret. What's done is done. And you just have to accept the way it is and move on.
But that's life. Things happen, whether you purposely make them happen, or the universe presents these things to you. And you just have to go with the flow, try not to get overwhelmed, let things be, and lastly: Have no regrets.
Everything happens for a reason.