Regret.

Some say that the worst feeling is sadness, or disappointment, or not feeling at all. That isn't true. Regret is the worst feeling anyone could possibly feel. While revenge is a dish best served cold, regret, is the sick feeling in your stomach when revenge takes action. The dead butterflies churning in your stomach. You want so bad to get rid of it. It's torture. It haunts you in your dreams and in your waking moments, flashes of images in your head that you can't shake off. Regret is wanting to turn back time and fix your wrongs, and realising that it isn't an option. You're swept away by the tides of time, reluctantly moving forward while occasionally stealing glances behind you. Regret is whispered pleas for forgiveness. Regret is a wrong that can never be righted, only a wrong that can be accepted when you help yourself find closure. Regret is the feeling you shove into a room and lock the door on. There is no cure for regret, because regret was created by your wrongdoings, and will only go when you leave this world, and even then, regret will be your only company to the afterlife.

Alternate ending:

Regret is a limitation of our vocabulary, the torn pages of our dictionaries. That is why, after all these years, I can say only one thing to you: I'm sorry.