You need to let her recover from the ill fitting saddle before you can ask her to work again, IMO.
You told me the saddle was the problem, and if you're ridden her for the past 8 months in the saddle, she's going to need time to recover and for her back to heal.
Can you get a massage therapist out, or a chiro? (not 100% positive it will work, but when you have a sore muscle, doesn't a nice massage help?
I think it would be best to get a trainer and work with them on a daily or as close to that as possible. You aren't going to get anywhere if you don't see her everyday (or close to it)
I would also forget about everything that you used to do with her. I think that gets into your head a lot and you really want to do it, but when you try, things don't go as planned. Pretend you just got her and you have to go through everything again. It may help you out.
But, putting your relationship with her aside, are you happy? Do you enjoy working with her? Do you look forward to seeing her?
If you are wanting to do more, it may be best to part ways with her and get a seasoned horse.
Personally speaking, even though I didn't sell Vega, i still did part ways with her. And getting Montana was the best thing to happen for me. I can do whatever I want with him, and It doesn't feel like i'm riding some old horse who I can't train. Not only does he still have a lot of energy, he was trained and ridden western a majority of his life and this whole english tack I put on him, is basically brand new to him.
Also, if you don't want to sell her right away, maybe think of leasing her out to someone and then you lease a horse... leasing a horse might give you your confidence back and you'll realize that you can train her.