Yes, I understand this completely (and am in a similar exhausted, anxious state myself). The thing that I find really helps is to tell myself 'This is how I feel today. It is just today. And tomorrow may be different. Just get through this day.'
It's so easy to let your thoughts spiral madly when you're in that state - the brain-spiraling is sort of what led to the end of my relationship, and from what I've read of your relationship breakdown it sounds as if it might be similar. Try to change your surroundings, maybe, if you start to feel that way? I force myself out of the house, if I can, to go get a coffee, or take a walk, or sit in the park... anything to give my head a fresh setting, and surround myself with other people.
It's also super easy to forget that your ex is dealing with other stuff that is not your relationship (and that men are generally speaking better at compartmentalizing this stuff). I mean, he's probably thinking about mundane stuff, and most likely when he thinks of you it is a high point, a lovely brightening thing for him to think of.
My friend always told me 'Focus on what is real, and what he is telling you. Don't let your imagination fill in the blanks.' I so wish I had listened to her because my own paranoia contributed a lot to the end of my relationship. So I'm telling you to do the same. You're doing SO well in letting him do the contacting. You're SO in control. Just sort of maintaining your position, don't swerve off wildly in any direction and undo your good, steady work.