OP, you sound like a good person and an even better friend but there are just some people who do NOT value that type of loyalty. I do not know your situation, but what you have physically described is depression. You have given away to much of yourself to this person and that person has given you nothing back and now you are left feeling drained. You must rise from the ashes in which you currently sit. Forget how you got there, forget who helped put you there, because just look around ARE they anywhere to be found?
You put your faith into someone else that was not worthy of it yet, this happens. Maybe you saw potential where others could not. But their failure is not your failure and even if you think it is, I will ask you this...is our success in life a measurement of how high we have risen? Or rather it is our success measured from where we have fallen... You are most certainly at a low point, but there is only one direction to go from here.