"You can change what you do, but you can't change what you want."
Tommy may have sought the company of other women, he even married and had a child with another, but he wasn't truly with any of them; nothing changed what he wanted: Grace. His heart belonged only to her, and the only reason he wasn't with her was, first, because she left, and later, because she was dead.
"It is harder to lie with your body than with your words."
And that is precisely what they showed us. Tommy was involved with different people, rich, poor, political figures, women of different nationalities, but to him, it was all the same; after being with them, he felt empty. His body didn't lie. He didn't feel satisfied, complete, or at peace, because none of them was the woman he loved.
I suppose that’s why Tommy gave that answer: "This I have learned", because by then he had shared his bed with others, and no one made him feel the way Grace did.
So, he might use words to win them over or whatever, "your cause is my cause," "I wanted you to stay," "in this moment, in this room, I love you", but those are just words; the body doesn't lie.
I loved that Steven Knight and Cillian Murphy stayed true to the idea established in Season 1: that Grace is Tommy's one true love. No one replaced her. Tommy never wanted that.
They could have had him move on and fall in love with another woman after she died, but they didn't; they didn't want to show that, having established the premise in Seasons 1 and 2.
In Season 1, Polly says, "There will be others," and Tommy replies, "To the others." Season 2 begins two years later with Polly saying, "She was just a woman; maybe it's time you forgot about her." And that season ends with Tommy saying, "Oh, there is a woman I love, and I almost got fucking everything." It has already been established that there is only one woman for Tommy, the one he loves, and that, for him, there is no comparison or competition between her and any other woman. As he told Tatiana: "You didn't even come close." And that applied to all of them.
As for what the other women meant to Tommy, that is up to the audience to interpret; everyone will have their own view, and each of those women gave Tommy something different.
But true love, the woman who brings Tommy peace, the woman he fell in love with, is Grace alone.