When I struggle with my voice, it's usually because I feel worthless ... sick and tired of soldiering on - my emotions are all over the place. I am tired of having to act a certain way and sick of trying to please that I no longer know how it is that I am suppose to be, or just giving in to those things inside my head that may not even exist anymore.
Of course this in only my take ... your spin my be entirely different. I try to look beyond the physical manifestations of anxiety because I already know that that is ... and in this case it's a shaky voice that simply won't go away. My explanation given, pretty much sums up how I am feeling when I struggle like so.
Good luck with working out what makes yours shake.