I know nothing about the Alexander Technique, but what I've gleaned looking at a couple of web sites, I'd say that it looks well worth investigating—which I intend to do forthwith.

In the meantime, I concur with the advice to make an appointment with an otolaryngologist (ear, nose, and throat doctor). Then, pending what the doctor says, look up a good voice teacher (the doctor may even be able to recommend one) and take some singing lessons.

I assume that you have not had any formal voice training ('scuse me if I'm wrong, but nevertheless. . . .). A good voice teacher can show you breathing techniques that will not only support your voice, but help to make stage jitters pretty much a thing of the past. Also, the teacher can check out the way you sing and spot any flaws in your vocal technique (and anyone who sings has a "vocal technique" whether they know it or not) that could be causing you trouble, and show you how to correct them.

You don't have to take years and years worth of lessons. Just enough to develop a sound basic technique that will help you preserve your vocal health and keep on singing well into old age. And don't worry. Taking some voice lessons will not make you sound like an opera singer. There are lots of folks out there who would like to be opera singers and wish all they had to do was take lessons. Unless the vocal apparatus you were born with has the potential of developing into an operatic type of voice, it just ain't gonna happen.

Good luck!

Don Firth