She posted a shot of herself standing onstage -- during the "Celebration" tour -- under a giant photo of her namesake mother, Madonna Louise Ciccone ... reflecting on the last time she saw her right before she died from breast cancer in 1963.

Her mother was only 30 -- Madonna was 5 -- when she passed away.

Madonna said for 81 shows on her tour she stared up at the sepia-toned pic of her mom ... wondering what she was thinking when she waved goodbye from her hospital window -- a moment Madonna didn't realize would be the last time she'd see her mom.

She says, "Nobody told me my mother was dying -- I just watched her disintegrate mysteriously, and then she disappeared, and there was no explanation except that she had gone to sleep."

Madonna says she asked for her mom's blessing every night of her tour ... and in doing so, brought her closer to her own brood of 6, whose pics she also shared in the carousel of Mother's Day IG pics.

She wrote, "I became very close to my children on this tour. We clung to each other in the long hours we worked and struggled. They helped me manifest my dreams."

The post naturally gave way to Madge reflecting on the trials of motherhood ... acknowledging there was no manual or university to parenting, but more a process of trial and error.