If you start to cry, you use a tissue. Take a few with you just in case - you may not even need them.
In the day or two leading up the hearing, do whatever you can to treat yourself well and put yourself in the best mindset as possible . . . sit in the sunlight, try to get some decent sleep, watch some ridiculously funny YouTube videos.
You very likely will NOT fudge things up . . . you have an attorney who knows the ins and outs of SSDI hearings and claims. It's a process with its own idiosyncrasies, so let her take the lead.
If you become distraught, take a moment to compose youreself. Breathe. If it's appropriate, apologize for your lack of composure but explain how desparate you are to get better and how much you need the Court's assistance and how appreciative you are for the Court's time.
Did you have an opportunity to discuss your concerns with your attorney?