צ֥וּר יְלָדְךָ֖ תֶּ֑שִׁי וַתִּשְׁכַּ֖ח אֵ֥ל מְחֹלְלֶֽךָ׃
Von dem Felsen, der dich zeugte, warst du unaufmerksam und hast Gott vergessen, der dich geboren hat. .
Rashi on Deuteronomy
תשי means THOU HAST FORGOTTEN. But our Rabbis explained it thus: whenever He was about to bestow good upon you, you provoked Him to anger and, as it were, weakened (תש) His power so that He could not do you good (Sifrei Devarim 319:3).
Sforno on Deuteronomy
צור ילדך תשי, even you, Yeshurun, the one blessed with erudition, once you turned your attention to the physical enjoyments of life on earth, undermined, weakened your wisdom and your knowledge about the greatness of G’d and His Torah.
Kitzur Baal HaTurim on Deuteronomy
You disregard. The letter yud, with the value of 10, is small, to imply that Hashem gave you Ten Commandments, and you tested Him with ten trials, and also to imply that Hashem should remember Avraham whom he tested with ten tests.