You will not go crazy no matter what you do. I am sure of that. Your therapist's idea is a good one to try. Here are some addition steps:
Facing your anxiety is not easy, but nothing will happen as a result. Try using your other hand to switch off the light. In order to do that, tell yourself repeatedly several hours before you try that you can turn off the light with any hand you choose. It is ok and nothing will happen. Repeat that, or similar comforting thoughts, as often as you can. This helps restructure your thinking patterns.
Remember that OCD is mostly a brain imbalance. It may have some to do with behavior and past history, but if you think of it as not part of you but caused by something else you can separate yourself from it. The messages you are getting from your brain are not right. They are wrong. And they do not have control over you. Keep that in mind whenever you feel the urges to do something related to OCD.
Do not allow the OCD to have any importance. Minimize the thoughts and feelings related to it. OCD is not who you are, it's only a thought pattern you are used to. You have the willpower to change it.
The more you think these thoughts, the more you can change the OCD.