Change it by thinking of good things. Having said that, sometimes you just need to allow yourself to be sad and treat it for what it is, a common and acceptable human emotion and not necessarily something that indicates a deeper problem - as is often the case these days.
What would you do if I gave a needless and rambling answer to a simple question?
I'd be grateful, and probably give them all my free time helping them find better jobs. But if they will constantly impose that fact in order to unjustly manipulate me, I'd pay my debts then just leave them.
What would you do if you feel like the world already left you behind in everything, in career, in relationships, in life... and you can't do anything to catch up without causing yourself more harm?
There have been times when I could feel nothing at all; not fun.
What would you do if I proclaimed myself the king of the Internet and demanded that you all worship at my hairy altar? That's not meant to sound naughty.