Whenan employer won't hire you full time because of a disability?

I work a minimal of 15 hours a week and I know that isn't much. I am on SSDI and since 9 months are up for a trial work period, Social Security contacted me for Substantial Gainful Activity. I do have to be supervised in this little office and I know I do not do the work the others do at all. I feel scared to death becasue I do not want my benefits taken away and at the same time I am trying to earn a little money for medicine and food. I feel like I am between a rock and a hrad place. Should I quit my job and really be very poor. I only have a 130.00 check a week from my job.