I am so sorry to hear about what you're facing. There are a number of "How to Be a Good Witness" guides out there. This is a pretty good one:
http://www.pacwcbt.pitt.edu/Curriculum/205%20Safety%20Perm%20%20Well-Being%20Pt%20III/Handouts/HO%209HowtoBeaGoodWitness.pdf
You sound extremely stressed. How well do you know your doctor? You can go to him and ask him for a prescription for half a dozen Xanax. I have to do public speaking about 6 times a year and my doc (bless him) will give me a few Xanax. Take one several days before you testify to see how you react to them.