Brenda is right. You are anything but useless - scared, sure, uncertain what your role is, why not, but useless no way. Stop beating yourself up, it's a waste of energy and unwarrented by the situation.
If you haven't already done so get some Ativan for yourself to take the edge off all these raw emotions. I think you really need it right now to help you deal with this. A year ago I got some excellent advice from seasoned members to make time just for me - REGULARLY - as Brenda is suggesting. Step away from being the caregiver for an afternoon, an evening, whatever you can swing and do something for you. Screw the housework, it will still be there when you get around to it. What good is a clean organized house if you're a mess?
You must take care of yourself first in order to be a help to your Mom. You are not responsible for everything and everyone, nor can you make it go away or go right. It's an unrealistic expectation and you will quickly burn out by trying to live up to an impossible standard.
If Mom finds something she wants to eat right now, that's fine. She's under stress too, let her handle it the way she wants. The doctors are aware of her cardiac status and they'll watch it. You do not always have to be the messenger here.
Give yourself a break.
Regards JoAnne