Fortunately I'm a girl so y'know, it doesn't really show..

speak slowlely and be calm as possible , too try to speak topics you know them well .

It happens the same to me.

You start speaking faster and don't pay attention to your English.

In those situations I try to speak slower to keep calm and recover the control of what I'm saying, but it's quite difficult when the subject gets you very excited!

Tell them about your feeling. Talk about it and you'll find yourself calming down gradually.