Telling letter writers who already submitted that I am not applying to graduate school
I had neglected to research the PhD students recently admitted to the programs to which I have applied.


Please don't compare yourself to others.  There are other, more productive comparisons to make, if you really need to make comparisons.

Compare your transcript with the course sequence at the schools you were going to apply to.  Do you have all, or almost all, of the prerequisites under your belt for your first two semesters?  If there are one or two courses missing, can you take those in winter, spring or summer?

It's often easiest to get a good start to graduate study if one doesn't significantly delay beginning.

I understand that you are concerned you may be being overambitious at this stage.  But if you received or are about to receive a Bachelor's degree, with good understanding and good grades, let that be your can of spinach to help you keep going with the applications.