For innumerable evils have compassed me about; literally, for evils have gathered upon me until there is no number (comp. Psalms 40:1, Psalms 40:2). The exact nature of the "evils" is not mentioned; but the worst of them appears to be "the deep and bitter consciousness of sin" revealed in the next clause.
Another was, beyond all doubt, the continued animosity of enemies (Psalms 40:14). Mental and bodily weakness may have been added, and have completed the crushing load whereof complaint is made. It is noted that tile exceedingly deep consciousness of sin here displayed "belongs altogether to a late part of David's life" (Canon Cook).
Mine iniquities have taken hold upon me, so that I am not able to look up; rather, so that 1 am not able to see. An actual failure of sight seems to be intended (comp. Psalms 6:7; Psalms 31:9; Psalms 28:1-9 :10).
They are more than the hairs of my head; i.e. they are more in number. Therefore my heart faileth me; i.e. "my courage" and "my strength of mind" (comp. Psalms 38:10).