I think the other ladies gave you some great advice
I'm wondering myself what this woman adds to your life? It sounds almost to me like you are talking about two entirely different women... One you care deeply about and treasure your history together, and another who is high-maintenance and moody. Is she maybe suffering from depression? Has she always been selfish, as you mentioned? Or has something happened in her life to change her?
I also would like to commend you for going to the gym! I would encourage you to not let your different schedules get in the way of that. I would recommend talking with her and saying you would like to share healthy support in each other's lives, even if you need to pursue your paths separately due to scheduling conflicts, or even when your energy levels don't match up.
Also, your rant is not unimportant! Having struggles with a long-term friend who is your fitness buddy and close enough to be one of your maids-of-honor is a big deal. We are here to support each other, so please don't poo-poo your problems