Could you tell me a time you felt sad?

Unfortunately, it's quite an easy one for me to answer at the moment because quite recently I really let someone down. Someone I had been with for the best part of four years, absolutely loved every second of it and then one morning you wake up and everything is different. Whole world gets flipped upside down, didn't feel the same way, all very sudden and after about a month it hadn't changed. So had to call it a day, as otherwise it would just be unfair for everyone involved and it was pretty horrible. The worst part was really feeling like I had let her down, I think that's the part which made me the saddest. When you let someone down you feel like you can't do anything about it, which is the horrible part.