For most people the tips given up thread will help like rehearsing, practicing with small groups etc and the more you do it the better you will get and the easier it will become.
There are some people, however, who have suffered from complex PTSD for whom this won't necessarily be the case as basically your brain has been wired incorrectly.
I had tried everything that has been suggested up thread and for many, many years nothing worked and I never improved, despite extraordinary efforts on my part
I don't think many people realise that, like the person who gave up their university course, being unable to do public speaking can have devastating consequences for your career and life and has caused some people to commit suicide.
Eventually after many,many years of psychotherapy and reading and working on myself I am able to present to large groups of people.
It isn't just about knowing your subject, it's how you perceive other people and the world around you.
If you suffered trauma through abuse or neglect as a child your brain develops defence mechanisms to make you disappear to avoid being seen as that could lead to death.
Unfortunately these defence mechanisms which helped keep you alive as a child severely limit your life as an adult as part of your brain thinks that being seen and being the centre of attention puts you in grave danger.