Wider tires at a lower pressure if they'll fit in your frame. Maybe sit more upright and pedal slower.
I don't think adding suspension to the front end will be worth the cost even if it does help your condition. Try out and purchase a bike that already has suspension on it.
If part of your issue is from a bad concussion you may have gotten in the accident, then that might take months to get over. I didn't have pain from my concussion, but I did feel like I was in a fog for over five months afterward. And during that time I didn't like laying on my back or being upside down .