I think your best bet would be to put your horse in training while you are healing.  Then when you are healed and starting to ride again, get into lessons with an eye to getting your seat very, very tight.  Perhaps lots of lung line lessons and riding without reins and sturrips.  Then from there, start riding horses that are slightly more difficult and work up to taking lessons on your horse.  The idea with getting your seat tightened up, is when you do start riding your horse again, if she does try to get you off, she'll find it much more difficult.  You'll be able to ride her out and hopefully she'll be more willing to be a good partner to you.   This may take a couple of years. 

Good luck.

 Spotted Pony

Hey spotted pont that is sort of what our plan is for now. My trainer has been working with me and Lily since I first bought her. She is still riding her 3 days a week and training her. She takes Lily out for me to groom her since I don't have full use of my arm. I can't ride until spring so I plan on riding my trainer's horses walk/trot to get my seat back and then do lessons on the lesson horses. If I do decide that Lily has mellowed, I don't plan on riding her for a year. My trainer wants to really work on her before we decide if I will keep her.

Wow - after reading some of these posts...  My 'story' is almost not worth telling, haha!  I have a hot spooky mare that I've had for several years now.  She has only thrown me once & I couldn't tell why.  She slung me off her back like she was tossing a ball.  I landed flat of my back, had the wind knocked out of me and was knocked unconscious when my head finally caught up with the rest of my body.  I had trouble moving, in general, for about two weeks.  I felt like I'd stepped out in front of a Mack truck - I've never had my whole body hurt like that in my life.  The 'pop' sound that my head made when it smacked the ground made me a true believer in wearing those "stupid looking helmets".   With that being said, I know my injuries could have been much, much worse. 

 There isn't a person on this forum that can tell you what to do with this mare of yours.  It's a tough spot.  I guess my first line of thinking is influenced mainly by my own dealings with a spooky mare.  I would suggest looking at some sort of variance in your training/riding.  Maybe this horse needs to run/lunge off some nervous energy before you get on her.  This has helped a lot with my mare.  Maybe she's just got your number and knows what works to keep her from working.  There's really no end to speculating what might or might not work for her. 

 Are you in a position that would allow you to 'retire' her and buy another horse more suited to you?  It's hard to hear people say , "This horse isn't for you", but sometimes it's the truth.  Weigh the pros & cons, and decide what it's worth to you.  If she's throwing you in such a way that will break your collar bone, she can also break your neck.

Sounds like Lily has a lot of issues related to being a PMU horse (abuse issues). Think about that industry, how horses treated like a product, or are just things.  The therapeutic riding center where I ride, they have a PMU gelding, but he has such abuse issues that only women, and kids pose less of a threat to him. He isn't even being used in the program. He is at the instructor's house. It could be that there are bigger issues than just saddles, or lack of saddle pads and saddle fit with Lily. That PMU gelding I mentioned does not like a certain rider, like heavy weights. Neither does he like men.

But you can't keep killing yourself over this horse's problems. Is it possible to let your trainer ride Lily until she is more mellow? Or wouldn't it be better if you had an older schoolmaster? Your trainer is a lot more experienced and can handle Lily better than you can especially since you have told us that your instructor did pretty good on Lily in a Dressage competition. 

Getting hurt is expensive, not to mention the problem of being afraid. I can tell you I won't ride a horse that bucks me. I have been bucked off once and that resulted in a long hospital stay. I have bucked one other time, but I kept my seat. Then, I never rode that horse again. I'm 55 now, and I won't get on a horse that I know bucks. All those injuries I got when I was 19 are now haunting me. That initial fall is why I have Fibromyalgia. So that's something to think about, what if repeated injuries cause health problems on down the road.

Six years is not all that long to be riding, in the grand scheme of things. Your body is probably just developing decent balance.  Considering your mare is young and spooky, you've done remarkably well.  You can thank your horse for teaching you some pretty important muscle memory that is going to serve you well when you get on the next horse, if that's what you decide to do. Good luck.