It is very easy to see that your consciousness, your "you", is nothing but chemicals and electrical impulses. Just have a few drinks, hit a blunt, or take any of the cornucopia of mind-altering drugs. The fact that a mere chemical, and in many cases, not very much of that chemical (Ativan, for instance, is usually prescribed at milligram levels - that is one thousandth of one gram) can completely change your personality shows that your personality is mere chemicals. If your consciousness was some sort of metaphysical "spirit" or "soul" it would be unaffected by physical things.
This also includes injury/disease, by the way, and I have living proof of that right here in my home: My mother is in late stage Alzheimer's disease. Alzheimer's is a progressive brain disease in which the brain is essentially slowly dying. It has already taken away very much of mom's personality. She is somewhat aware, and she has some older memories, but most of who I've always known as my mother is already gone. Her body lives on relatively healthy but Claudia has left the chat. What little there is left of her will fade away until she is a zombie, and eventually that too will pass as the part of her brain that governs her body (heartbeat, breathing, other biological function) finally dies.
As @KevinReijnders has said: Yes, psychological issues exist that aren't necessarily cured through medication, but this only proves that some things just can't be fixed (or we haven't yet found the right medication). If I drain the oil out of my car's engine and run it until it starts knocking, it will not be fixed by simply putting the oil back in, even though oil is the "medication" that makes engines work properly. The engine is damaged and cannot be fixed with chemicals. It would require a complete teardown and replacing parts (crankshaft, connecting rods, bearings, anything else with damage), and even then there would be collateral damage, parts that were damaged but still function somewhat - in other words, even after repairing the engine it would never be the same. It would always have those parts that had damage but still function, and those parts could still fail at any point in the future.
We have the technology to rebuild engines. We do not have the technology to rebuild human brains. The fact that we can't do it, though, does not mean that it can't be done. Some time down the road somebody could invent a 3-D printer that can deposit neurons in exactly the right places and re build "you". That's science fiction now, but it might not always be. Storing trillions of bytes of data on a computer chip smaller than your pinky nail was science fiction at one point too, and now you can buy a 1tb Micro SD card for sixty bucks.
