What is the meaning of ‘add insult to injury’?

There are times when things don’t go according to the way we had planned. We feel bad about it, and in order to snap out of the terrible mood we are in, we pay a visit to our friends hoping that they will cheer us up. Sometimes, instead of helping us overcome our depression, they succeed in getting us even more upset. The idiom ‘add insult to injury’ means to make a situation that is already bad, worse.

First of all, he was driving on the wrong side of the road, and then to add insult to injury, he proceeded to abuse me for driving slowly!