Hi Olivia,
As the others have stated, it is very normal to feel the way that you do. I am out 10 years from my cancer, but only one year out from my last op to remove pre-cancerous polyps - has been a long journey - and although it has been a big improvement for my future health (physical) I have found it has provided a mix of feelings during this last 12 months. I too had big "it's come back" feelings in the first 4-5 years, every little cough, loss of energy (probably due to not enough exercise) and that sort of thing, but that feeling has definitely faded to a great extent. It doesn't matter what any professional (ie. doctor, nurse, etc) says, the feelings take a long to to fade, I have found that if I acknowledge the feeling and then put it to the back of my mind, then I can live with them. After 10 years the feeling of the cancer returning has been locked firmly away.
So, don't feel like you are inadequate or that "you shouldn't be feeling like this", it is all very normal.
As Samex says, this site is a fantastic place to "normalize" what you are feeling and dealing with.
Good luck and lots of kind thoughts,
craftyone