Tiff, never worry about being too nice as that's really just not possible to be too nice.
Life has its own system of rewards and punishments and, as the old saying goes, "What goes around comes around." When you're young you don't really believe in that because you never get a chance to see it but as you get older you see it happen time and time again. Sometimes its almost instant and other times it takes years but it does. So people who do those things to you will have their own price to pay at some other point in time. Your challenge will be to remain kind and graceful when you do learn about their comeuppance.
Also, I've learned to be thankful for anything or anyone who reveals to me a side of someone that I couldn't see. These people can save you a ton of heartache... in your case you had women take loosers off your hands.
Just learn to adjust your perspective and see that their actions and behavior have nothing to do with you and your being too nice. It's their behavior based on their own values.