Please help me interpret the poem's meaning. What lines are particularly striking in "The Nature of an Action" by Thomas Gunn?

In this poem, Gunn describes a search for meaning and selfhood, using images of rooms. In the first stanza, he leaves a room that he is "fond" of, but which he also describes using heavy, confining, static images: "heavy-footed chairs," a glass "loaded" with "wax pears and grapes" (wax fruit is static). A polished table is described as "holding down" brackets, a mantelpiece and a "marbled" book. The word marbled refers to a type of paper that once lined books, but also doubles as yet another image of heaviness: marble is a heavy stone, a stone used for headstones. 

So following the desires of his heart, the poet leaves the room, but only steps into a "corridor," another confining space. The corridor is "narrow" and he finds himself in it for twenty years. It is "bare, dusty and hard," all unpleasant images, but the real problem, the poet recognizes, lies not with the corridor but within himself. His "habits" have made him an "obstacle" to his own quest for meaning and fulfillment. He finds himself running up against the "great obstruction of himself." 

During his time in the corridor, he finds a "passive illness" in himself that is characterized by self-doubt. Significantly, he repeats the word "doubted" twice: he "doubted" the corridor was real, and more importantly, he "doubted" himself. Doubt saps his will.

In the third part of the poem, he finally musters the will to act. The most significant part of the poem is the couplet that opens this final section: 

"My cause lay in the will, that opens straight/Upon an act for the most desperate."

He finally, though this act of will, has found the sense of selfhood that enabled him to open a door instead of wandering full of doubts in a hallway.

He finds the room he enters almost identical to the room he left twenty years before: it has the same heavy-footed chairs, wax fruit and marbled book. But even though it has not changed, the poet has changed:"Only my being there is different," he writes. The word "only" is ironic in this context: his change is the all-important crux that changes everything else.

In the end, Gunn says, our outward environment matters far less than our inward self. At the same time, the poet leaves uncertain as to what exactly has changed internally for him. We take his word that life will now be different--and perhaps his word is all that matters.