I have personally played off and on since 2004. For a two year period I tried to practice for at least an hour every day. You will have good days and bad days. My advice to you is...if you are having a bad day on the theremin, just turn it off and come back tomorrow. This will help you from becoming too frustrated, think it's too difficult and possibly lose interest.
I currently own a Kees Enkelaar MK-II (no longer in production), a standard Moog Etherwave, a Wavefront suitcase model, an Moog Etherwave Pro (no longer in production), and a Keppinger tube theremin (no longer in production).
Please do not infer from that I am a world-class player. I can play quite well, but not well enough to justify posting videos or playing in public. It is not from a lack of skill or technique but soley from my impaired sense of hearing and a less than ideal sense of pitch. But, since I'm the only one listening, that doesn't matter so much.
BTW...the particular amp and speaker used with any theremin can also have a significant effect on its tone.