As Oldtemecula said, a right-handed theremin (which is standard) will have the pitch antenna on the right and the volume antenna on the left when you approach it. In this instance, the problems are definitely manageable. If you are used to your left arm being your pitch arm, however... we may need to have a different discussion. But first, let's assume you'll be controlling volume with your left and pitch with your right.
With an immobile middle finger on your left hand, you'll be okay to play. Truly refined theremin playing is as much in the volume technique as the pitch technique, and there is a lot of nuanced finger movement involved, but much of it can be accomplished in the wrist and arm. Even then, other finger movements will still produce dynamic effects. Some very accomplished thereminists, in various ways, actually keep their middle finger extended a little beyond the rest of their fingers when playing so they are better able to control dynamics when playing softly. So really, working with your circumstances may actually provide you some advantage in the (re-)learning process that other thereminists do not have, as it's normally something people consider much later without realising how much it helps their sound.
Tl;dr: I think you can absolutely do it if the situation is as I understand it. In fact, you may even be able to turn it into a great strength, as it resembles a dynamic technique most beginners do not learn until later. I hope you're able to rediscover and enjoy the instrument once more!
