I think there are a number of different things going on here -- for one thing, the way you phrase this ('friend I've stuck by turning on me') suggests you think she owes you gratitude for your longterm fidelity.
But remember, she doesn't see it like that -- from what you say, she's too socially unaware to have been conscious of the fact that her social behaviour is offputting and tone deaf and that you've been forbearing for years. She just thinks you like her, and is not conscious of any debt of gratitude that you've put up with her for years and, in your view, you've established her in a social group she wouldn't have had without your sponsorship.
The question I think you need to ask yourself is why you persisted in your relationship for so long with someone who monologues incessantly, and clearly takes very little interest conversationally in you? What was in it for you?
I ask because it seems to me that people who persist in befriending someone whom other people think is awful, is because, consciously or unconsciously, there's a power element. 'Aw, she has no one else.'/'Aw, she's a good person underneath it all, and I'm the only one who can see it' etc.
For whatever reason, you put up with her poor manners and monologues, even when other people remarked on them and kept away from her, possibly in part because it was a one on one friendship. Now suddenly she's included in a new social group, and she's around other people with you on an equal basis -- suddenly her bad manners are being more obviously exhibited, because she doesn't have the social awareness to cover up what appears to be possibly happening in her head: 'Oh, I'm more interested in these exciting new people than in boring old @Badenergee, who's as familiar as an old slipper.'
It also possible that someone who is presumably unused to socialising in collective situations, that she has the social nous of a seven or eight year old who ignores one friend on a collective playdate in favour of the more exciting one -- that she's just not used to normal social group manners, like 'don't play on your phone when one person is talking, don't turn your back on someone who's answering your question, even if you're not that interested in the person'?
There's also the more unpalatable idea that you were fine when you were all there was, but now that she thinks she has other friends, she prefers other people to you? (As, in fairness, you clearly prefer other people to her...?)
What you should do probably depends on what you want to happen. Is she likely to piss off this new social group? From what you say, you've only been out twice with them, so it's early days. If she does, do you want to retain the friendship of the group? If so, I'd be focusing on establishing friendships with the others, and making it quietly plain that you don't come as a pair, in case you're ditched along with her, if they get tired of the monologues. (It sounds from what you say as if her talking prevented you from making other friends at baby groups?)
Also, are you done with her as a friend? Do you still see one another one on one? If she's ditched by the new group, and starts wanting to see you as if nothing happened, what do you want to happen?