Two years ago next month, my husband of 17 years left me for an older woman (I am 43, he is 47, she is 61), his parting comment was "I hope the cat is ok and you don't starve her to death", (he was the one who always fed her), that night, I had the kids and Billie in my bed, the kids and myself had a fitful night, Billie didn't move, from the moment he left, she became lots more friendly and loving and started to come back in the house, where before she was happier outside.
After about 3 months, the kids said to me they thought Billie was a lot happier now "he" had gone.
He hardly ever see's his children, despite living within 30 miles and has kept the same Dr who he visits once a month, Billie has since died and we have got Cloud, Shadow and Misty and whenever I have the misfortune of having to speak to him, he will always ask how the kittens are before he finds out the welfare of his children.
Time
does heal, you won't see it now, but honestly, I
promise you, it will get better, give Mia all the cuddles you need to share and I am sure she will help the situation for both of you to settle into a new routine.
Good luck.