I'm helpless.

I'm in a relationship with someone I've known for a long time, he has obvious relationship issues from the beginning.
Now we're getting further and further apart. I feel alone and broken. 
 I can't eat or sleep, I'm shaking and it hurts physically.  Even breathing is difficult for me. 
I don't know what to do with my mind.  I'm so afraid of losing him and then being alone, I feel like I can't stand it anymore.
I ask myself what is so wrong with me that everyone always leaves me, I am in therapy, working hard on myself and yet I am always alone and the only person who always seems to lose him is me.
What do you want me to do?  I can't do it anymore.
I don't have anybody to talk to right now, so I'm just gonna leave it out here. I hope that's okay.  I'm just so helpless now.