RG, why would you say such a thing? You're not stupid! At least you've never sounded that way to me.
No matter how long we work at something, or how hard we try, we are still going to have days, weeks, or sometimes even months when nothing seems to go right. If a patient lies to you, that's not your failure. From what I hear, stuff like that happens quite a bit in the medical profession. Not even a brain surgeon can read minds to tell who's lying to them.
It sounds to me like you want a profession where you can help people. Being a nurse, being on the FD. Those are both pretty noble callings. Don't sell yourself short. There's a lot of people in this world who depend on givers like you. Hang in there. No slump lasts forever, and this one won't, either! It'll all come back to you then.