"Nagging doubt" refers to a persistent feeling of uncertainty or suspicion that keeps bothering you. It is a constant worry or suspicion that continuously nags at your mind. Full definition
In these uncertain economic times where American families put an ever - higher premium on a good education, and with nagging doubts about flaws in funding in the public education system, the education reform movement has become a red - hot button. (usc.news21.com)
But although I knew that the interventions on my first birth had been unnecessary, there was a horrible nagging doubt in my mind that I might not be able to progress my labour without the use of drugs. (homebirth.ie)
They speak of missing time with family, struggles to complete lesson plans and nagging doubts over whether it's worth the sacrifices to stay in their profession. (ncforum.org)