Janet Howd's Answer:
An insecure voice is often a sign that the material being presented has not been properly thought out or is being pitched at the wrong level for the audience concerned.
From what you say you are avoiding those pitfalls but you don't mention having given any thought to how your everyday voice is supposed to react when it's suddenly expected to lift your ideas off the page, project them into listener's ears and lodge them in their minds.
The education systems most of us go through place a lot of weight on achieving good hand to eye coordination and so do a pretty good job of making sure that setting ideas down on paper becomes second nature. But because talking - with an occasional shouting match thrown in for good measure - is something we can do even before we go to school, mouth to ear coordination is mostly left to fend for itself: and mostly it does until we expect the impossible.
To put your voice to the test in an actual performance without any training is madness and your muscles, quite fairly, balk at the task. They need to be trained.
When we want to speak out over sustained periods, tongues have to work much harder than usual to drive words to the front of the mouth; jaws and lips have to open a lot wider than usual to allow the extra-resonant sounds out; chest muscles have to expand far more than usual to allow room for the faster airflow a voice needs if it is to produce higher volume and longer strings of sound.
Choose and memorise two minutes of your presentation. Get out into parks or fields or along sea shores or in streets full of heavy traffic – anywhere, in fact, where you won't feel self conscious about talking loudly into your cell phone.
Walking briskly, speak out your chose script in any accent other than your own. Use Italian, French, Spanish, Caribbean, German, Russian, Aussie or Kiwi, Chinese, Japanese, Indian, extremely posh English or any other regional accent from the UK or USA.
Don't worry about whether anyone else would think you're getting the accents right. Just play with this exercise and discover how your own mouth and body respond to the patterns and sounds of other people's way of saying things.
The value of this game of pretence on your phone is that certain accents created predominantly in the front of the mouth with an open jaw are going to make your voice much louder than usual; some - because they are produced with smaller jaw and lip opening will muffle it; other accents formed far back in the mouth will make you sound hoarse; some will be strong on nasality; all will pronounce words in ways that are different from your own speech pattern. and your voice will be picking up on these differences all the time you are speaking.
After ten minutes worth of this part of the game, try a different tack.
Still walking briskly and speaking out strongly - but this time in your own accent - add different qualities to the words you are saying. Say them harshly then calmly; invitingly then with distaste; warmly then coldly, kookily then seriously, etc.
Fool around like this for another ten minutes then go home in silence. [NB. Throat aches after these first attempts are to be expected]
Go through this twenty-minute routine every day for a couple of weeks but vary the script, the accents and the qualities so that you don't get bored.
Hopefully you will find that playing around with sound, words and body in this way will wake up long unused vocal muscles and their postural support muscles, and make you aware that presentation is a physically as well as mentally demanding task.