You are very right about being concerned. Going into a $20K hole is nothing to sneeze at! To pay that back (including interest payments) could take you upwards of a decade if you aren't disciplined and really think it through.
IMO and some quick googling The average salary for a dental assistant isn't enough to justify going into that amount of debt.
Specifically, in a worst case scenario, say that you make $32K after graduating and getting a job:
and it sounds like you have some credit card debt to clear up.
All in all, the $32K runs out pretty quickly and you have the bare minimum left to pay the $20K school loan back.
This means that over time you may end burdened with the loan for several years. Not Good!
I would recommend to try and find a job that would pay for it. It doesn't have to be in the dental field just as long as you can get tuition reimbursement for going to school. There are many companies that have that offer that benefit.
Also, get rid of that credit card debt ASAP. Makes me cringe when I hear of people carrying a balance :(.