You do not want to take more than ativan or xanax in combination. For what you need these for, combination dosages are not needed. Indeed, I have seen people who were terrified of flying, fly on a plane using Ativan alone, and they were literally terrified of flying. (Yes, about an 30 minutes to an hour before is the usual protocol.)
And you need to know that these people, now medicated, were functional.
The problem previously as I see it was your dosage and the fact that the medications you were given may have dropped your blood pressure in addition to "relaxing" you. Too much medication is as bad as too little and you should always, always take the medication as a trial dose a few days before the presentation to see how you feel.
Sadly, because biochemistry is different in all people we cannot say what a reaction will be like 100%. But, I can say that Ativan or Xanax tend to be excellent choices and are well used for the purpose you wish. If it was me, I would try, with doctor's permission of course, one of these medications alone, and not in combination with others. One is definitely enough for anxiety reduction and presentations. Steven