Although the risk of respiratory depression even with 20mg extremely low I'd stick to 5-10mg which is the standard medical dose for this type of anxiety often given at this dose as an anti-anxiolitic before surgery.
Maybe start with 5 and if it hasn't done what you want from that then in an hour or so add another 5mg....
With no tolerance, if this hasnt reduced your anxiety then it probably isn't going to..I wouldn't go above 10mg