Invisible walls

Invisible walls stand tall, impenetrable.
I should know.
At first glance, I may seem open, defenses down,
But come too close and I'll shut you out.
Every time.
As friends, there are no fixed boundaries, my heart is always open,
My life an open book.
But go too far, and you will hit solid glass.
So unexpected that I don't always know it's there...until I find myself hiding behind it once more.
A heart will do many things to protect itself,
Often bypassing even conscious thought.
I've seen the tears that come with "love", the abuse of power, the loss of oneself.
My heart is wary, my defenses strong.
So much so that no one has ever broken through,
Even when I first believe it otherwise.
Deep down,
I'm stuck in a fortress all my own,
Hoping for the day I can let it go,
My heart free to trust completely.
But until that day, I'll stay here,
Behind my last defenses: safe and whole.
At least as much as any one person can be.