Rameses, I am so sorry you are going through this. Detachment is HARD, the process of detaching from someone you loved is similiar to the grief process.
You broke off the engagement for a reason right? Trust your instincts.
I call what you're going through "meltdown" days and they do happen. But as time goes by there are less meltdown days and things DO get better.
That feeling of "doom" is it anxiety? Some things that help anxiety (besides going to a doctor for medication, which isn't a bad idea either) are exercise, healthy diet and some form of meditation or prayer. I manage my stress by running or walking almost daily. I feel MUCH better after a three mile run.
I'm very fortunate because I have a good friend I can call and I ask her to talk me through it when I have those days. She tells me this WILL pass and I WILL get through it.
So let me tell you... . this WILL pass and you WILL get through it!

you can do this!