If you feel bad about not being able to help, just explain to whoever asked that you don’t have the time to do the quality work they expect or that you have other, more important deadlines to meet.
Start by determining how much work is ideal for you and then be sure and confident to decline any offered workload that goes beyond it. If you feel bad about not being able to help, just explain to whoever asked that you don’t have the time to do the quality work they expect or that you have other, more important deadlines to meet.