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Six months after Fitz left office, Mellie filed for divorce. She could no longer be trapped in a loveless marriage, especially when he was eager to leave her. She couldn't take it anymore. She would have done it sooner but it would have looked back for the President and First Lady to end their marriage immediately after he was voted out of office. Fitz was planning on doing it himself, but she did it first out of spite. The look of shock on his face when he was presented with the papers had been priceless (she secretly took a picture on her phone). The press had a field day, but Mellie was past caring. She was ready to start her life over again and begin her political career.
Jerry and Karen supported her – their relationship was just starting to recover from years of Mellie pawning them off on multiple nannies – which she was grateful for. And for some reason, Teddy worshipped the ground she walked on. She was confused about that, but didn't question it too much.
Mellie was also grateful when Olivia finally kicked Fitz's ass to the curb and started a real, honest relationship with Jake Ballard. They actually developed a friendship afterwards and bonded. It was pretty nice, even though Mellie wasn't sure she could ever forgive either of them.
Life was better after the divorce and the old Mellie slowly began to return. She wasn't miserable anymore and that helped out a lot. She could never be who she once was, of course, but that was fine with her. She was older (and wiser?) now. Love wasn't even a priority since she was too burned to even think about another relationship. Plus, she loved her independence.
Mellie Grant is actually happy and that is fucking amazing.