You're too Nice

I'm 24 and my entire life I've heard "You're to Nice." Once during a job review that was listed as one of my downfalls....
"You're too Nice."
I am that person on a job interview that lists being nice as a bad quality or says "I care too much."
I admit being nice has screwed me over a few times throughout my life but its also brought me perks that "mean" people didn't get.
I think I am too compassoinate for my own good - I don't stand up for myself. I think because I was picked on as a kid I just figured that if I just went with the flow things would be better and that's true occasionally....
The problem is people see my niceness and then take advanatge of it and in my niave mind I don't see it. Fast forward 6 months I am friendless.
How do you stop being too nice?? Am I supposed to just start yelling at people?