Is your choir disturbing your organ practice? Do people stare at you and you feel uncomfortable? When somebody comes up to you and asks you something, do you mess everything up in your hands and feet?
Try practicing in a noisy place. Put some music or radio on in the headphones. Ask choir members intentionally chat a bit.
It seems contrary to the common sense but noisy environment can help you get more focused. If you approach it in the right way, of course.
You need to put all your willpower into your playing.
This is not easy. At first the distractions will be difficult to dismiss.
And you shouldn't. Don't fight them. Don't engage in a negative self-talk.
Just be in the current measure you're playing. Especially with your eyes. Don't allow them to fidget.
If you persevere, you'll have no worries during real public performance.