If you absolutely need the job to survive, try talking to the manager. You said that being assigned as floor person is "just not going to work out." But, you do OK at the register, right? Why not tell the manager you feel as though you're a good fit for the register position and ask if you can be assigned to that position, either more often or from here on out.

The manager can either say yes or no. Try approaching him/her as a concerned employee who wants to do a good job. Sometimes managers are more receptive if you ask for their advice. I do this all the time with my boss: "Do you have a minute? I'd really like your advice about something that's been on my mind..."

If the manager isn't willing to work with you, or if you still feel like the position just isn't going to work out (no way, no how), start looking for a new job. I know, easier said than done.

The bottom line is that you need to take care of yourself. Talk to your doctor. Talk to your T. Talk to the manager. But, don't feel guilty for taking care of yourself, even if that means quitting this job and looking for a different one. The store will survive with or without you, so make the decision that's right for you.