You’re probably stuck with having waived your claims. That’s the hard part about severance agreements; you have to make a decision on waiving potential legal claims based only on what you know right at the end of your employment. There really isn’t a meaningful opportunity to do a full investigation, so it’s a risk assessment. 

That’s another reason why consulting with a knowledgeable employment attorney can be helpful before you sign it—his or her experience can really go a long way to determining the likelihood of there being a claim if you had access to all of the information that you needed to review first. That way, if you do decide to sign it, you can be more at peace with the decision.