Adam Clarke on Psalms 18:23

I was also upright - The times in which David was most afflicted were the times of his greatest uprightness. Adversity was always to him a time of spiritual prosperity.

Mine iniquity - Probably meaning what is generally termed the easily-besetting sin; the sin of his constitution, or that to which the temperament of his body most powerfully disposed him. What this was, is a subject of useless conjecture.