I understand what you're going through. I'm having a similar problem right now. My ex is everywhere I look (for diverse and depressing reasons).

If you really think that it's not a healthy relationship for you and it provokes such internal conflict, just remember that ultimately you can only make someone else happy if you yourself are happy. He may not be able to see it, but by stopping yourself from being in this relationship you're doing both of you a massive favour in the long run.

Just try and keep your relationship strictly professional. Make sure you don't have any communications outside of the office (whatsapp is a poison when it comes to trying to get over someone) and if you do communicate in the office it's only over work related themes or harmless chit-chat.

If you're sure about it, then stick to your guns and remember you're doing the right thing,.

Best of luck! And I hope that as the conflict/situation calms your symptoms start to go away. But as healingme4me says, maybe go to the doctor's just in case!