What do lines 126 through 158 in Macbeth show him thinking about?
I assume you mean in Act I, scene 3 where Macbeth and Banquo have just received news that Macbeth is now Thane of Cawdor.
Macbeth now sees that two of the three prophesies have come true. Now all that is left is for him to become king. He is wondering whether or not he should kill King Duncan to get the throne or if he should wait and see if fate will bring it to him. At this point he is facing temptation. He is nowhere near the paranoid and crazed man he becomes after killing Duncan.
"This supernatural soliciting Cannot be ill, cannot be good: if ill,
Why hath it given me earnest of success,
Commencing in a truth? I am thane of Cawdor:
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not."