Paltrow, 47, revealed she tried to silence her doubts but they never completely went away.
She wrote: "I tried to convince myself it had been a fleeting thought, that marriage is complicated and ebbed and flowed. But I knew it. It was in my bones.
"At first, I was moderately successful at turning the volume down on that knowledge. It would be years until we said the words aloud. But, that weekend, a dam had cracked just enough to hear the unrelenting trickle of truth. And it grew louder until it was all I could hear."
And Paltrow - mother to Apple, 16, and Moses, 14, with her ex-husband - revealed she and Martin, 43, actually split up about a year before they announced to the world via her goop newsletter, that they had "consciously uncoupled".
She said: "When we made a commitment to approach our separation this way, and about a year before we introduced the phrase to the world, we put it to the test. It was hit and miss. We had great days and terrible days. Days when we couldn't stand each other, but forced ourselves to remember what we were aiming for. I worried about the world finding out that we were no longer together before we were ready to say it. And how to say it? What to say?"
Paltrow tied the knot with second husband Brad Falchuk in 2018 but revealed she will always have a special place in her heart for Martin, who she divorced in 2016.
She said: "I know my ex-husband was meant to be the father of my children, and I know my current husband is meant to be the person I grow very old with. Conscious uncoupling lets us recognise those two different loves can coexist and nourish each other."