am I under any obligation to keep quiet about the settlement terms?
No, unless some relevant information is missing in your description.
I gather that the case is tried under the New Jersey law. If otherwise, please clarify. Also, I'm assuming that you did not sign anything that gives your attorney(s) unrestricted, full discretion in the decision-making.
From the description you provide, the settlement seemingly violates the New Jersey Rules of Professional Conduct and therefore it is void.
You mention that the settlement agreement was reached between the law firm and the defendant. Since you deem the terms simply unacceptable, it appears that the attorney(s) representing you violated RPC 1.2(a) (stating in relevant part: "A lawyer shall abide by a client's decision whether to settle a matter.") and/or RPC 1.8(g) ("A lawyer who represents two or more clients shall not participate in making an aggregate settlement of the claims of [...] the clients").
The court in Cardillo v. Bloomfield 206 Corp., 411 N.J.Super. 574; 988 A2d. 136, 140 (2010) states that
"A contract that violates the Rules of Professional Conduct is void and unenforceable as a violation of public policy. Jacob v. Norris, McLaughlin & Marcus, supra, 128 N.J. at 17, 607 A.2d 142. Because the Cardillo Agreement violates RPC 5.6(b), it is not enforceable."
Replacing RPC 5.6 with the rules I mentioned above, a similar conclusion would apply to your situation. Therefore, you are under no obligation to comply with the terms or confidentiality of what is an invalid settlement contract.
P.S: We pro se litigants are often ridiculed for representing ourselves in court, but the situation you describe is an example of (many) attorneys disavowing their rules of "professional" conduct and defrauding even their own clients.