Thou shalt forget the shame of thy youth; rather, of thy maidenhood; i.e. of the time when thou wert a maiden, before by the covenant of Sinai Jehowth became thy Husband (Isaiah 54:5). The "shame" of this period was 'the Egyptian bondage.
Israel's later condition would be such that the very recollection of this bondage would fade away and cease. The reproach of thy widowhood. Israel became a "widow" when Jehovah withdrew his presence from her, when the Shechinah disappeared from the temple, and the temple itself was destroyed, and Jerusalem was a desolation, and the people captives in a far land.
The special "reproach of her widowhood" was the Babylonian captivity, with the sins that had brought it about. This too would be forgotten in the good time to come, amid the glories of the Messianic kingdom.