Regret

Regret is one of the most horrible feeling one can have. It is akin to the emotions one feels after the loss of a loved one.

Regret is not being able to run fast enough and missing the bus that's parked only a few metres from you.
Regret is watching the love of your love marry her soulmate, not knowing if you could be the groom right now if you had just tried.
Regret is feeling that consumes your entire being as you cry for the death of a loved one, knowing how much pain you hurt them with, or what you could have done to make them smile once more.

It is wearing a nice dress on unexpectedly rainy day. It is spilling milk and dropping snowglobes. It is staying up till 12 to type out a blog post, when knowing you'll feel like hell tomorrow morning.