What can I do to get my body to stop rejecting the pain? [I Am Not As Strong As You Think I Am]

When you share one heart with someone, the only way to separate it.... Is to break it.
I feel like my brain has known that we broke up, for a while now, but seeing that text, somehow made a connection from my brain to my heart.... And it just shattered!
I can't think about anything else... I have barely been able to function all day! I guess he just took away my ability to deny that he just fell out of love with me. No....
This void in my heart physically hurts in my chest. My stomach is in the worst knots, my chest keeps getting tighter and tighter. Like something super big and heavy is on it and the outer cavity keeps shrinking around it like one of those space saver bags that you vacuum all the air out of... Its hard for me to breathe, if I am not consciously focusing on my breath, it is short, shallow and constricted, or I just stop breathing all together! I keep finding myself tightening random muscles to the point where they get really painfully.
What can I do to get my body to stop rejecting the pain?