For mine iniquities are gone over mine head - This is merely an enlargement of the idea suggested in the last verse - that his present sickness was to be traced to his sin, and that he was suffering the punishment for sin. The idea is here that his sins were very numerous and very aggravated. They had risen up around him, or had so accumulated that the mass rose, like waves of the sea, above his head. A somewhat similar idea - though the thought there refers rather to the number of sins than the degree of guilt - occurs in Psalms 40:12 : “Mine iniquities ... are more than the hairs of my head.”
As an heavy burden ... - That is, they are so heavy that I cannot bear them, and my frame has sunk under them. This might mean either that the sense of sin was so great that he could not bear up under it, but had been crushed by it (compare Psalms 32:3-4); or that on account of sin, “as if” it were a heavy weight, he had been crushed by disease. The general idea is, that the real cause of his sickness was the fact that he was a great sinner, and that God was punishing him for it.