fear of fainting....

Hi,
I am glad to have found this forum.
I am a teacher so it might seem odd that I have developed this fear. I am actually a very good public speaker and have given many good lessons and asemblies. I have never enjoyed it however - and would rather not do presenations in front of collegues - the kids dont worry me. I am being pushed to go for promotion but do not want to as I will have to do more speaking in public. In the past I just prepare well, swallow my nerves and get on with it.
My problem is this. About a year ago I suddenly felt as if I was going to faint whilst giving a big presentation in front of staff and students. I wondered if it was the effect of beta blockers which i take daily for migraines(oddly enough thisis propanolol which is prescribed for some to help with public speaking fears!!). I adjusted the timing that i took them and it seemed to work for a while. I thought they may have lowered my blood pressure too much?) Then the sensation reoccured - accompanied by a huge panic and desire to end the presentation and get away which i could not. i have made adjustments like having something to lean against in case I feel dizzy - but obviusly my train of thought is affected and I sound like a bumbling idiot for about 20 seconds whilst I regain my composure. i can then continue and return to my usual confident self. I cannot get away from having to do presenations but I am now terrified as it seems to me that i only have to think aout developing the fainting sensation whilst public speaking and it develops. In other words i am making this happen. I feel a sensation of panic with it. I generally have low blood pressure and wondered if this might be the root. I am sick with fear for the new term starting and just want to hand my notice in. Any ideas?