Hi Heartbroken,
I’m in a similar position and am about to write a post soon. You have to try not to get wrapped up in something that is pure fantasy. Do you know for sure he is on a date? Why imagine him on one if it causes you so much pain? You are literally creating this pain out of thin air – there is no basis in reality.
As I mentioned I’m in the same position, so I am not judging or criticizing you. It might help if you imagine YOURSELF on a date with some new guy (not with him!!). And as Anita said, don’t contact him!! When you feel an urge to text him just go do something else for an hour – go to the gym or take a long hot bath or start reading a new book. When you’ve finished that activity the urge hopefully will have passed. Or if you want to contact him, imagine how the interaction will conclude. Most likely it will not end in you getting back together and it will only cause you more pain, so try to remind yourself of that.
I know how incredibly hard this is. I’m in your shoes now and I’ve been here a dozen times before. It will get better, but only if you give yourself physical, emotional, and mental distance from him.