When I kept silence, my bones waxed old through my roaring all the day long.
Here begins the main body of the psalm, detailing from his own case the grounds why the guilelessly-confessing and consequently forgiven sinner is to be regarded as "blessed" (Psalms 32:1-2).
I kept silence (in contrast to Psalms 32:5) ... waxed old. When I tried to stifle the voice of conscience, and would not confess my guilt, my bodily frame was so affected by the torments of conscience, which would not be stilled, that it became powerless as that of an old man (Proverbs 17:22). His "silence" does not imply that he did not pray at all, but that he did not guilelessly (Psalms 32:2) "confess" his special, "transgression."
Through my roaring. The "silence" as to his sin stands in contrast to his "roaring" (literally, that of a lion) or loud moans, because he found no ease of mind or body, through the stings of conscience acting on his frame.