"Dependent" and "independent" for financial aid purposes are defined by the US Department of Education. Your college is just following the rules they have to follow for every student. There's no form that I know of that your mother simply signs and makes you an independent student.
This is from the FAFSA FAQ:
"If you answer No to every question below, you are considered a dependent student and must provide your parents' information.
If you can answer Yes to at least one of the questions below, you are considered an independent student.
-Were you born before January 1, 1987?
-Are you married?
-At the beginning of 2010-2011 school year, will you be working on a master's or doctorate program (such as an MA, MBA, MD, JD, PhD, EdD, or graduate certificate, etc.)?
-Are you currently serving on active duty in the U.S. Armed Forces for purposes other than training?
-Are you a veteran of the U.S. Armed Forces?
-Do you have children who will receive more than half of their support from you between July 1, 2010, and June 30, 2011?
-Do you have dependents (other than your children or spouse) who live with you and who receive more than half of their support from you, now and through June 30, 2011?
-At any time since you turned age 13, were both your parents deceased, were you in foster care or were you a dependent or ward of the court?
-Are you, or were you an emancipated minor as determined by a court in your state of legal residence?
-Are you, or were you in legal guardianship as determined by a court in your state of legal residence?
-At any time on or after July 1, 2009, did your high school or school district homeless liaison determine that you were an unaccompanied youth who was homeless?
-At any time on or after July 1, 2009, did the director of an emergency shelter or transitional housing program funded by the U.S. Department of Housing and Urban Development determine that you were an unaccompanied youth who was homeless?
-At any time or or after July 1, 2009, did the director of a runaway or homeless youth basic center or transitional living program determine that you were an unaccompanied youth who was homeless or were self-supporting and at risk of being homeless?"
There are very detailed FAFSA instructions here - http://studentaid.ed.gov/students/attachments/siteresources/CompletingtheFAFSA10-11.pdf - and some description of the process to seek an exception to the dependency definition here - http://www.fafsa.ed.gov/faq002.htm#faq002_3 .