I would ask for an itemized accounting from them as to all payments made and why they are contending that you owe additional funds. You should also do an itemized accounting and use the loan amortization schedule as an example as well. You then can compare your calculations with theirs. If you can not reach an agreement with them and they are still off, I would consult an attorney for a free initial consultation and talk with the attorney about the benefit of writing them a letter on your behalf to address the discrepancies
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.
With the little information that can be communicated on this forum, I cannot say whether this is or is not legal. However, I can say from what you have posted that I have very real concerns that you this is not legal.
Mr. Leroi gave you very good practical advice. Stand up for yourself, and ask them to cooperate with you in understanding where all of these charges and fees came from. In order for such charges and fees to be legitimate you must have agreed to them in the loan contract or they must be authorized by law. If the company refuses to cooperate, you may want to consider filing a consumer fraud complaint with the Colorado Attorney Generals Office or consulting a civil trial attorney for a free consultation.
I hope this answer provided some helpful guidance.