While there may be some affirmative defenses that you can use, I can't think of any from reading your problem.

As far as getting proof of payment, do you have any bank statements from that period? I would suggest bringing along every bank statement from when you made your last payment through today (well-organized, of course; don't just bring in a pile of paper).

However, I don't think this debt is past the statute of limitations. I may be wrong, but I believe the SOL for litigation is six years in Colorado. If your original agreement had a "choice of law" provision that specified a state with a 3-year SOL (New Hampshire, for example), you'd want to find that contract and bring it as well. Whether or not a judge honors the choice of law clause is anyone's guess, but it's worth a shot.

Good luck.

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I am not a lawyer, nor am I an "expert" or "authority" on anything. This post reflects my opinion only, and may be wrong or inappropriate. Always consult a legal professional for guidance.