I'm afraid there is not much you can do now, if you are completely convinced that the medication has done something to you brain, you could try a brain scan. I was given a brain scan to see if I had any tumors, to try and explain the sudden change in my behaviour/personality.
Other than that, if I were in your shoes I'd just follow the usual course of depression - doing everything your psychiatrist tells you to do, performing different techniques to grasp ahold of yourself. I remember when I was numb, I had to do this thing where I focus on my senses for 30 minutes a day (in 5 minute intervals). Basically, I'd focus on sight, naming off everything that is the colour blue, then I'd focus on smell (lighting a match for example), touch, sound, etc. I'm sure you know what I'm talking about if you've seen psychiatrists, but yeah - you just need to get out of the slump your in before anything.
Sorry I can't give you the answer, but if I were you I'd stop focusing on the reason and focus on fixing the problem before anything.