What’s considered not feeling well and therefore you’re allowed to break your fast? In my case I’m at work and I’m weak and finding it hard to concentrate.
Fast days are not easy. It is pretty much normal to feel weak and hard to concentrate. In order to break the fast the person has to feel sick, or has a condition that will render him weak, such as a nursing or pregnant woman.
Although fasting is not easy, it is one of the ways that we mourn the destruction of the Bais Hamikdash, and the mourning of the Bais Hamikdash is one of the things that will give us the merit to see its rebuilding, bmiheira byameinu.