Themes in Genesis are not becoming more apparent. Here, the same barrenness present in Sarah is present with Rebekah (25:21). This will not be the last time we are reminded that the promises of God are fulfilled not in the power of our human flesh, but in the power of the Holy Spirit.
The desired outcome is not so much to despise yourself, but to hear in the gospel the invitation for the barren to come and experience fruitfulness.
Isaiah 54:1–3 (ESV)
1 “Sing, O barren one, who did not bear;
break forth into singing and cry aloud,
you who have not been in labor!
For the children of the desolate one will be more
than the children of her who is married,” says the Lord.
2 “Enlarge the place of your tent,
and let the curtains of your habitations be stretched out;
do not hold back; lengthen your cords
and strengthen your stakes.
3 For you will spread abroad to the right and to the left,
and your offspring will possess the nations
and will people the desolate cities.
My powerlessness is an invitation, even a promise, to come and to trust God for fruitfulness in my life!
John 15:5–8 (ESV)
5 I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. 6 If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fire, and burned. 7 If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you. 8 By this my Father is glorified, that you bear much fruit and so prove to be my disciples.