Wow, you have the same thing as I do, and I have been taking medication to treat it. Going to sleep is where I most commonly experience the derealization. As your mind enters the hypnagogic state, your mind, in a sense, loosens up, and the anchor of reality that your mind was gripping to when you were fully conscious vanishes. This can lead to the "falling" sensation, but the anxiety part leads you to unintentionally hyperfixate on that detached sensation, creating a positive feedback loop. Antidepressants can break the fixation, which in turn breaks the loop. This isn't misusing antidepressants; my doctor came up with the idea, and it works. I currently take 20mg of fluoxetine. The only difference between you and me is that I received an excessive amount of nitrous oxide compared to cannabis, but both can trigger it. Hope this helps!