Please don't feel like a failure! Sometimes we spread ourselves a little to thin and something is going to suffer for it. In this case, both you and the organization are going to suffer. Decide what your priorities are and then go from there. You really need to listen to your body and symptoms and take care of yourself. If you don't want to step down, is there something else that you can give up? Take care of yourself and by doing that you will make good choices.
All the best!