A couple of suggestions:
1.) As other people have said--long, slow, deep, rhythmic breathing can calm you. Try picking a calming word (peace, grace, calm, etc.) to repeat to yourself as you breathe.
2.) Right before you go into the situation, take a minute and tighten every single muscle in your body. Work really hard at it and tighten as hard as you can, hold it for a count of 5, then release all of the tension.
3.) If you find the urge to cry coming on, and you absolutely MUST not, bite your tongue or cheek as hard as you can. It's not pretty, but the pain may distract you from the urge to cry.
4.) If you have some time, process as much of it as you can now. Allow yourself to cry, scream, hit a pillow, etc. as much as you need now. Don't stop until you feel like stopping. Crying triggers a very powerful chemical response in your brain--it's why people suddenly feel calm, numb, or even a little sleepy once they cry themselves out.
5.) Try to think through all of the possible outcomes of the situation and how you would handle them. Most of our anxiety comes from the unknown. If you can think about the scenarios and start to come up with a plan, you might feel somewhat calmer and more in control.
Good luck to you. I really hope everything turns out OK.
