Cheating.

There is absolutely no worse feeling in the world than finding out you've been cheated on. Personally, I'd rather be straight up broken up with than have to find out my partner was seeing somebody else behind my back. You spend time and effort and you put yourself out there for a person until they gain your trust, and then they rip it all from your hands in an instant. It's worse than just losing a significant other. You lose part of yourself. You lose the ability to trust others in fear they may take it and shatter it all over again. But if you find yourself in the situation where you've been cheated on, think about it this way: If they cheated on you and you were willing to give them your all, they actually cheated themselves out of true loyalty. It's their own loss more than yours. They made this decision on their own, and although it may hurt at first, you'll be the one that comes out on top in the long run.