The Romans 4 text, particular verse 19, is referring specifically to the moment of conception not Abraham’s entire life. The complete verse reads:

And being not weak in faith, he considered not his own body now dead , when he was about an hundred years old, neither yet the deadness of Sara’s womb

Abraham was 100 and Sara was 90 and Isaac’s conception, though miraculous, still required normal conjugal relations. With the relationship turbulence that had gone before (remember, Hagar and Ishmael were still around) and obvious biological deterioration due to age (it is likely that Sara’s menstrual cycle had stopped), Abraham and Sara could both have faltered. But, instead, believing God they engaged in normal relations which resulted in a miraculous conception.

Therefore, we understand that Abraham became the father of faith not because he never faltered but because he never quit.