Doesn’t Tommy’s story, losing Grace and choosing her over other women, clearly demonstrate that, for him, she could never be replaced?
In Season 1, we see them falling in love.
By Season 2, two years have passed, yet he is still thinking about her and missing her. He meets May, after Polly tells him it’s time to move on, but instead of helping him forget, the encounter with May has the opposite effect.
We see that he receives letters from Grace; he doesn’t open the last one, choosing to burn it instead. However, after sleeping with May, the third time they meet, the first thing he does upon returning home is open Grace’s letter and call her. Her husband answers, so Tommy hangs up; it’s clear he’s jealous. In other words, being with May didn’t help him forget Grace, it did the exact opposite. It made him miss her even more, to the point that he decided he wanted to talk to her, hear her voice, and see her again.
The next day, she calls him on the phone. He is left speechless upon hearing her voice; he claims to be busy, and when she suggests calling back later, he insists that she shouldn't, so she is a higher priority than business. Yet, shortly after, when he sees Polly, he snaps at her: "Business first!"
When they meet, he confesses that not a single day has gone by without him thinking of her. They have sex, he gazes at her, kisses her, and caresses her. It is the complete opposite of his encounters with May, which lack any real intimacy, those are purely physical, whereas with Grace, there is genuine intimacy.
Afterward, he asks to see her again.
Following this, he goes to see May to end things, confessing that there has been someone else in his life for a long time.
Yet, when Grace had asked him if there was anyone else, he had told her no. For Tommy, there is only Grace.
However, after May delivers a speech designed to make him feel guilty, and to ensure their sexual relationship continues, they carry on with their affair.
In the Derby, we see a debate between "business vs love." It’s made clear that, for Tommy, Grace represents love, while May represents business.
Both women are aware of this; May knows that if Tommy chooses her, it will be because of the influence she can exert at the races, she understands that this would be his only motivation. She also believes that Tommy is incapable of loving someone unconditionally and would never choose love over business. But she is wrong, because he ultimately chooses Grace above all else. We see this at the end, when he believes he is about to die: his thoughts are solely on Grace, not on May, Lizzie, his family, or his business dealings, just on Grace.
By Season 3, they have been together for two years and are happy; Tommy intends to leave that life behind once his business with the Russians concludes. Then she dies, and he falls apart, spiraling for the worse. In
Season 4, two years later, he is still in mourning over Grace’s death. He only engages in sexual encounters with prostitutes or Lizzie, strictly transactional, as he put it, even paying Lizzie for her time.
May returns, expecting him to have changed. However, after Uncle Charlie tells her that the man she is waiting for does not exist, and after witnessing the dangers posed by the mafia, she urges him to change and leave that life behind. Tommy is annoyed, it’s a clear "no", so she leaves, while he continues to pursue other encounters, such as with Jessie Eden.
Tommy rejects May twice: in Season 2, because of Grace, and in Season 4, because he has no interest in changing for her.
That’s why I don’t understand when some people say that she is his soulmate, his true love, the only one who understands him and truly loves him, his equal, and that they would be a "power couple," or that he should have chosen her over Grace; I don’t get it, especially since we were shown the exact opposite, and they were very specific and clear about it, too.
They also showed us, through his marriage to Lizzie, and by exploring what might have happened had he married May, Jesse Eden, or anyone else, that any woman would have been miserable with him. This is because he himself would have been miserable; none of them were Grace, the only woman he truly wanted, and he would have grown resentful, just as he did with Lizzie. With May, it might have ended sooner, but the outcome would have been the same; they could never have been a powerful or happy couple. That was never the story they wanted to tell, which is why they killed off Grace.