Your condition sounds truly terrible. You have my sympathy.
It would be deeply unethical for me or anyone on this panel to try to discuss your particular situation in a forum like this. I would hope that you have a trusted friend or relative with whom you could discuss the matter, someone who knows you well, and who could serve as a sounding board as you work toward what has to be your own decision.
I will only make this observation: you certainly should not attempt to make any decision about this if your depression is not being treated adequately.
What I can do as a professional philosopher is to steer you toward some academic work that surveys and discusses the ethical issues involved. I suggest you read Robert Young's entry on "Voluntary Euthanasia", which you can access online at the Stanford Online Encyclopedia of Philosophy: http://plato.stanford.edu/entries/euthanasia-voluntary/ The article is mainly concerned with the social policy question of whether voluntary suicide should be legalized, but I think you'll find the arguments discussed to be relevant to your own situation as well.
Again, I am very sorry you are suffering so greatly.