I had a bad fall last year where I was stuck on the ground for 2 months. When you get back on you have to trust your horse. They can sense when your nervous. Try some ground work. Free lunging works great for working with trust. Then when you decide to ride him/her make sure you keep the horse gathered up. Don't let him hang on the reigns but also don't let him hold his head up. Best thing I would say is try doing a lot of half halts with him! Keep him on his toes so he doesn't expect you to do transitions . Just think, shoulders like a queen. Keep ur butt farther back in the saddle so your horse doesn't decide to be a 'fat kid rolling down a hill'. This will encourage him to slow down a bit.
Just an idea that popped into my head from the Velcro, maybe use strapping tape to pull your shoulders back/ help with posture, it'll just make it uncomfortable for you to lean forward so you are encouraged to have shoulders back?
Someone I know uses something that Velcro straps around your back and shoulders and armpits and basically it prevents you from moving your shoulders forward. I don't remember what it's called, but if you like, I can look it up.
Maybe every time you feel unsafe go into a "safety position" like say in your head 'heels down, eyes up, sholders back' and say that in you head, also think about sitting on the back of your bum, this will make it harder for you to lean forward. Also leaning forward will encourage your horse to go faster to its best to take a deep breath and lean back. Also for confidence maybe try riding with a friend, this may help you focus less on what the horse is doing and relax more? I do eventing and wherever I'm worries about a jump I just think, heels down, leg on, shoulders back and eyes to the top of the fence and everything else will work out
I wish I could help with the shoulder but I don't have a clue:( however it's a great start that you've already got back on but take it slow and try doing some groundwork and bonding with your horse and just being around him should comfort you if you have trails where you ride and your horse enjoys them go for A nice evening trail ride to help relax and don't worry about anything:). Hope it helps