Hello Poseidon =] I'm not very familiar with problem horses, as I've worked with ponies almost all of my life (yes there is a difference in their behaviors and why they do things o.-) but I'm going to try to give you some help.
First off, it sounds as if Abby has decided that she's the boss and that you can't do anything to control her- and essentially, that is true- but ONLY if you ALLOW her to be the boss, and you don't have the confidence to tell her you are the boss. That's the first problem and something you can't just give up on. If you allow her to be the 'lead mare,' she's going to start messing with your confidence, your happiness, your love of horses, and really even your head if you let her. So that really needs to change. You have to want it to change though, and until you decide that you're going to stick to this until the very end, there really isnt even a reason to try. Try to get into the mindset ' I am the lead mare, you will listen to me. '
Respect starts from the ground up, meaning you need to have her listening when you're smaller than her before she'll listen to you when you're on her.
telling her that you're confident with your body language[even if you really arent ) is a key thing when you go to catch her, release her, walk her somewhere, treat her wounds, tack her, trailer her, etc. hold yourself proudly, and don't act afraid around her. If she gets too close to you, glare at her- staring right into her eyes and shoo her away. Tell her who you are and what you expect! Slumping, avoiding eye contact, and allowing her to push you around will make it even more firmly ground into her head that SHE not YOU is the lead mare.
At the moment it would not be a good idea to have her 'join up' with you, but once her leg is healed I HIGHLY recommend doing it. It will make her submit to you, and tell you 'ok, ok. You're the alpha.' check out How to Join Up With a Horse - wikiHow
to learn how if you've never done it before.
WHILE her leg is healing, just work on ground manners as far as not letting her crowd you. If she gets to close, push her away with your body language. Tell her 'hey, that doesnt fly with me. Cut it out.'
Drop the riding for now, until she's listening to you on the ground. THEN you can start riding her again- and don't GIVE IN if she acts up! Correct her correct her correct her. Don't let her stop until she does what you want. YES she is stronger than you and YES she will ALWAYS win a war that envolves muscle. You can.not.win a tug of war with a horse. You CAN however, will a wits war. Use your head- catch her off guard. Make her work! Try reading Four Ways to Teach Your Horse Respect
for help with riding, its helped me and my boys a lot.
I don't know if that helped at all and its awefully late... theres so many other things you could do that I havent even mentioned, but I need to sleep ^^ just remember that your ACTIONS cause REACTIONS. A horse will always take advantage of unconfident owners. If you can, seek professional help. Ciao and I hope you guys can get back into your rightful pecking order again, keep me updated!
P.S; please please please
leave the twitch as a last resort. Its rather cruel and can make so many things go wrong. If you have to, then fine- but tire your other resources first. Try graining her as you spray her leg!