Oh Emma, it's like you're in my head. Everything you've described is something I have struggled with or continue to struggle with. There have been 20 different times in the last few days I've wanted to talk to my bf, but I know I need to follow his lead and give space. Sure, it hurts, but I work through.
Limit yourself to 1-2 msgs a week and steel yourself that you may not get any response for a while, if at all. I know it sucks to see him on Facebook assume he's chitchatting to everyone, but try not to speculate.
Insecurity is my middle name. Take some deep breaths and stay busy. If you msged him Monday, email him Friday something lighthearted like a funny comic and the caption "this reminded me of you" and a "Have a great weekend!" and leave it at that til next week.
It's going to be OK :)