In the first two stanzas of the poem, the speaker asks God if He can possibly forgive the speaker's sins. He worries that God will not because he has so many sins.
In stanza two, the speaker is worrying about a couple different things in particular that he has done. He has been the door of others' sin. In other words, he has opened the way for others to sin by sinning himself. He has also wallowed in sin (laid down in it and enjoyed it, like pigs wallowed in mud).
In Stanza three, both of the images you mention simply refer to dying. The idea of spinning the last thread seems to refer to the pagan belief in the Fates, one of whom spun the thread of life.