This can be a valid contract. There is no requirement that you have independent legal advice prior to signing such an agreement. If your ex had legal advice that makes it less likely that she could try to set aside the agreement or argue that it does not apply.

Generally an agreement regarding property rights will be enforced, as will an agreement about parenting of children. Th court always has the right to review such an agreement and overrule it if it finds that it is not working in the best interests of children. And an agreement to pay less than the Federal Guidelines for child support can always be set aside by a court unless there is some other arrangement in place to make up the difference. The court can set aside a spousal support award if it was not clearly made to be a final award or if th agreement does not properly contemplate the various points raised in legislation regarding such support.

An agreement can be set aside also if there is evidence it is unfair or was negotiated in circumstances that are unfair, such as one party bing under significant mental distress at time of negotiating or signing. This is why it is a good idea to ensure both sides have legal advice, financial disclosure and so in so that it is harder to try and invalidate the agreement down the road.

Other than these exceptions, a separation agreement can be binding.