The reason as I understand it Alicent is so fuming and miserable is because she's been basically forced by her father, who she loves despite herself btw because he's the only parent she's known for so long, into a marriage that ostracized her only friend, ally and comfort at court in her obviously difficult teen years but that marriage has amounted to nothing. There is obviously no love or passion but neither is there duty because a woman, as its been drilled in her head, has to be just a womb and despite being loyal and dutiful her children have amounted to nothing. Not just in terms of being future kings but also the way they mean nothing to her past-memories addled, openly bias to his daughter, husband. She is trapped in a marriage with no love, fearing for her children because there is no one to stand up for them, having lost her best friend, who gets to live the way she wants, her only allies being her father who has always seen her as just a pawn and a family murdering cold blooded man who could rat her out for something she didn't want done (not counting the absolutely so far useless Cristin). Everyone comparing her to Cersei has to see that Cersie had the comfort of power, knowing her son would be future King. Once you start to peel off the layers on Queen Green, you will see an absolutely powerless desperate woman who's only source of happiness, Rhanerya, was stolen from her just so she could be a pawn in a game that now everyone hates her for wanting to win.
