Nothing had changed except for the fact that Abram received the promise of God that he would have offspring. And so, it was not Sarai's barrenness that led to Abram's failure, but the fact that Abram was reminded of her barrenness by the reiteration of the promise of God. The presence of Hagar contributed to Abram's fall by giving Sarai a candidate for a surrogate. That journey was a result of Abram's failure to trust in the providence of God. So, a remnant of that failure contributes to Abram's failure in this chapter.
Sin often has farreaching consequences, many times leading to more sin.
She excuses herself for her worldly plan, by saying in effect that God led her into it by refusing her children. Rather than waiting on the Lord and letting Him work, Sarai interprets her barrenness as God allowing her to take action. So, she decided to take things into her own hands. We often think that God needs our help, and so we try through our own means to accomplish His work, instead of waiting on His will. In both cases, we find people through their own efforts doing the work of God.