hi there

i don't think you should worry that turning down other folks shifts makes you look bad. a good employee, which you want to be, shows up on time for their own shifts and does a good job of them and covers when they can for GENUINE staffing problems/emergencies.

i think it's fine to draw the line between being a good employee and being walked all over and, due to the amount of stress it's giving you, maybe talk to a line manager/supervisor etc?