It is impossible to let you know whether the contract is legal and binding without additional information. As a general rule, a valid contract (whether oral or written) must include an offer, acceptance and valid consideration. Consideration means that both parties give up something of value in exchange for the other party signing the contract. It is a facts and circumstances test in order to determine whether a contract is valid. This means that an attorney (or judge) will need to analyze all of the facts and circumstances surrounding the formation of the contract to determine whether it is enforceable.
If you're concerned about the "contract" or your legal rights under it, you should consult with an attorney and discuss the rest of the facts surrounding the contract. Hopefully this helps. Good luck!
The first thing that I learned in my law school "contracts" class was that, "Past consideration is NO consideration." Consideration is a bargained-for exchange between the parties to a contract. In this case, unless you signed the Agreement in order to gain something yourself (such as peace of mind that your Ex wouldn't sue you immediately), there was no consideration for your signature on the document. It is just a signed document -- it is not a contract.
However, if you promised at the time that you would repay the money, or stated that you were just borrowing it, then you did have an enforceable agreement that your EX is now just requesting that you document. I have handled situations where emails, letters, text messages, and initial or partial repayments between partners or family members have documented an enforceable agreement to repay the money. The document that you recently signed becomes an acknowledgment, a ratification, and renews that initial commitment to repay the loan(s).
Moreover, to fully answer that question as to whether any consideration supports this agreement, a good attorney would sit with you and ask you a LOT of questions. Why did she ask for the agreement? Why did you agree to sign it? Consideration is more than just an exchange of money. Consideration can involve the exchange of promises that are sometimes understood between the parties. However, it cannot involve what is subjectively inside the mind of one party that was not objectively expressed and agreed upon.
There is also the question of duress in this, but I have a hard time believing that a person adult enough to have a fiance was actually "made" to sign a document, unless you were in a truly vulnerable position at the time. Where the two parties are of equal bargaining power, it is unlikely that a court would find duress sufficient to nullify an otherwise valid contract. This does assume that you are over the age of majority and also mentally competent at the time you signed the document. Duress cannot be merely that you didn't want to be hassled, or embarrassed by a court suit, or anything short of truly extraordinary circumstances.
I don't know if this is useful to you, but the main lesson is this: Your signature upon a document that you felt pressured to sign does not automatically equate a contract...but it may be, in specific cases.
Without knowing all of the details, reviewing documents, and interviewing witnesses, no person should assume that this Answer constitutes specific legal advice for any specific legal situation. No attorney-client relationship is created by posting general legal responses on this site.