I've ridden in the abetta endurance and honestly, I don't love the seat either! It's awful slippery, too. Something a little deeper (like a barrel saddle, or an aussie) might help! I have a barrel saddle with a suede seat/fenders, and while I hate that the suede gets stained by jeans easily, I stick like glue and the high cantle keeps me there.
Riding out a bolt is difficult, so it's important to do everything you can to prepare for one-- even if your horse seems like a quiet, safe ride. In the arena, practice making your horse soft to one-rein stops (allowing you to take away his head) and also practice feeling what it's like when he tenses up. Always pay attention to his ears, his head, and his muscle tone so that you can sense when he is preparing for a spook. When he does, knowing the one-rein stop and having a horse who responds softly to it gives you one defense against that spook-- you feel him tense up and you bring his head around before he can respond.
Other than that, just practice sitting deep. My horse tended to have spooky days before I established to him that I was a trustworthy leader, and so I trained myself to respond to every spook with a deepening of my seat.
You ought to see a doctor for your chest. :( You might need some kind of brace or support until you are 100% again.