ACS Education Services Student Loan Complaint

ACS Education Services Student loan department,

Non-federal student loan Dealing with my lender or servicer Trouble with how payments are handled Texas

I have been paying my student loan through ACS Education Services for 10 years. Never missed a payment, and have never been late on a payment until recently because we are going to pursue legal action. My principal balance has increased instead of decreased. All my payments are mostly applied to the interest and not the principal balance. I am on a FI-ED rate plan but the interest seems to be variable and calculated by which day i pay. ACS is saying i owe more than what i originally took my loan out for. How is that possible? In the end they want me to pay DOUBLE what my original loan amount was. How can they get away with that? I understand that some of the payment needs to be applied to interest, but the majority of it should not!