Wow! This is very powerful and vivid. I love the metaphor of a shield, and I definitely relate. It's hard to put your heart out there, after it's been trampled (by anyone), but it makes it harder for you to open it up the next time.
Your title and the word veins are misspelled, unless that was intentional. Other than that, this is definitely a favorite. :)
One of your best poems! Love the wording and how simple the message of the poem is, we can all relate to it and the fact that they hurt us and we put this shield up, which in turn only hurts us more because we do not let anyone else near.