The verse describes an enigma, something which is common, but no less remarkable for that. Solomon applies this series of human failings to himself, and this is the point of his list.
Sin has its own deadening effect on out hearts. Every time we give way to evil, we harden our hearts a little more, and the hard heart has less insight into itself. We see less clearly the sinfulness of sin, and we care less about it. God recedes further from us, and what he thinks of our lives troubles us less. It becomes easier to sin next time, and sensitivity to evil fades. Now, we need to drink longer and deeper of the cup of wickedness to be satisfied by it, for its addictive power gets hold of us, while at the same time it reduces its ability to satisfy. We are being swallow by the serpent who will not let us go.