I know exactly what you mean. I had it really severe from about your age to my mid-twenties when it decreased somewhat. I found it worse than social anxiety, and I had that severe too!
I found that using CBT-like techniques to suspend judgement and shift the laser beam attentional focus onto consideration of the wider context (which I can describe in more detail if you like) to be very effective, among other things.
A tendency to notice extremely small details is a perceptual difference some of us naturally possess that can predispose to such obsessive checking. Such detailed perception can be very beneficial in some aspects of life, but when associated with anxiety can cause massive distortions and magnification in our evaluation of risk. Another large contribution to such distortions comes from the media, which makes many diseases seem far more likely to occur than they do in reality. General stress levels and lack of distraction and wider perspective due to staying indoors too much were among my major triggers.