Do I know my GRE scores when I choose to send them on test day?
When you finish the GRE, at least a computer-delivered GRE, you will be asked if you want to report  or cancel your scores. If you choose "cancel," you don't get to see them or report them. If you choose "report," you get your unofficial scores, then are asked to report them.

According to ETS (emphasis added):


  On test day, after completing the test and viewing your unofficial Verbal Reasoning and Quantitative Reasoning scores, you will be asked to designate which score recipients you want to receive your General Test scores.


At this point, reporting is optional, you don't have to report to anyone at this point. 



As for whether you personally should reschedule or retake it after 21 days, that's a question we can't answer, but hopefully this information helps you decide.