Agreeableness, kindness, generosity, and empathy, are all good qualities. No one ever complains about the person who is accommodating. As a matter of fact, it's encouraged. However, no one ever warns the big-hearted person that people could exploit them for these qualities either.
Growing up, I was always the girl who was told she was nice, easy to read, and wears her heart on her sleeve. My heart was so big for everyone but myself. I wouldn't find it so hard to believe that mine could escape my chest, travel up my arms, and make it's way all the way down to my sleeves. On some level, I also knew that I was naive. People kept telling me I was, so it had to be true.
The issue with being a naive person is that you see the best in humanity, whether you do it consciously or not.
We are optimistic about people because we wouldn't hurt a fly. We naive people think that others will act the same way we do in a situation. We could never foresee or wrap our heads around how truly bad some people are. We see the best in people, so we often fall prey to their attacks. That is our Achilles heel because the truth is that some people aren't as kind as us. The problem with being naive is that you're too naive to know what to look out for until it bites you in the back. The only way to escape being naive is to go through a lot of bad experiences that shape you until you become jaded.
Humans have an amazing ability to adapt. If you hurt a certain part of your skin enough times, calluses are formed. Kind-hearted people usually get metaphorically stabbed in the heart a few times, until it forms calluses. They learn to become less agreeable, kind, generous and empathetic so easily. People who misbehaved with you and benefitted from your kindness will complain that "you've changed" when you finally give them the boot. Take it as a compliment, it means you're evolving into someone with a backbone. The worst thing that kind-hearted people can do is to take this evolution as a negative event. They wonder what happened to their old loving and trusting selves. They worry too much that they're jaded and that they lost something beautiful about themselves when they can't love so easily anymore.
They fail to recognize that in order to survive, humans adapt. If this is you, stop blaming yourself for treading carefully after tripping so many times. Precious things deserve to be protected. You've given your heart away to undeserving people so easily in the past as if it was on a plate. This time, let them work for it. The ones who are worth it will earn it from you. Don't worry if your heart is calloused. It's the same heart you've always had. It now comes with thicker skin, but it can love just as hard as ever. It's not broken. It's just more difficult for others to get to it now, and that's okay.