She probably enjoyed riding together just you and her, you're her friend and family and it sounds like she enjoys spending time with you.
However the dynamic changes in a group than just one on one, perhaps she doesn't like that or she doesn't like riding faster. Even if she can elect not to there can be a lot of unintentional peer pressure in groups. I know there are lots of people I like hanging with one on one but not so much in a group. Lots of people don't like groups of people, even if they know them, there is nothing wrong with that it's just the way they are.
You can stop inviting her on group rides and such but if she really is your best friend and you really do love her to death then make time for her. Ride with your friends when you want, but maybe once a week or twice a month you have standing one on one ride together - just because you like riding with others doesn't mean she does, she's tried it your way maybe you try it hers too. Friendship takes compromise, she rode with you when you couldn't ride with anyone else, maybe you can ride with her sometimes.