Your description sounds exactly like me.
I will never understand why there is so much negativity in the world. It seems there are a lot of people that can't handle when someone is nice to them and they respond with meanness.
I know how difficult it is to not let other people's negativity have a bad effect on you. Sometimes instead of trying to grow a thicker skin I try to place myself in their shoes to see if I can understand why they are so negative, sometimes it works and I walk away still feeling good about myself while understanding the other person's point of view. Remember that often you will have no idea what their problem is, so just tell yourself they must be having a bad day and leave with your own psyche undamaged.