I'd do a different test before you do anything else. This is a serious situation, and you want to be certain you have the facts. (Your lawyer should have told you this also.) It sounds like your ex is acting just the way you'd think she would act if she had been hiding the truth from you (or was hiding her doubts), but be certain, first. I would suggest you use Ravgen, it's the best DNA testing company out there.
Then if the new DNA test comes out the same way, you have to decide what role you want to play in the life of the child. Do you love him or her? Does the child love you and count on you? (Or have you not been around the child that much anyway because of your doubts about paternity?) If you've been a regular in the child's life, it's not just about you and the mom any more. If the child loves you and you disappear, you will have a long-term very negative effect on his or her life.
My suggestions would be:
1) Test again to be sure your anger is accurate. If the first test was wrong and you are actually the biological dad, try to address how mad you were at the mom and let some of it go. Keep in mind, some women genuinely don't know who is the dad, rather than trying to fake out some poor schmo. It's a shame she didn't have the courage to tell you way back when that there might be a question, but this is the reality of right now. I would do another, and very reliable, DNA test before opening any of that can of worms.
2) Try to decide what kind of relationship you want to have with the child if the second test also shows you're not the biological dad. If you don't want any relationship with the child, you can
2a) talk to the lawyer about trying to get some of the support money back (it might not be easy since you signed the birth certificate, and she might not have any money).
2b) if you do want to continue to have the relationship with the child, for the child's sake and or your sake, talk to a counselor, about how to make that happen. It won't be easy because you're so mad at the mom. But a 5-year-old's life is also going to be turned upside down by what you do, and in your shoes I would prioritize that before even your anger at the mom.
In fact, I'd talk to a counselor about all of it. You need to find a way to not feel ripped off, and a lawyer will be all about assuming you were ripped off. Do a re-test and then talk to a counselor, and decide what you want. Put a high priority on what's good for the child.
Checking back here. Did you do a re-test? What did you find out?