It's generally considered not a good idea to rasp the walls because it weakens what may be already compromised, but if you round the walls off, soak them as Appyt suggested, with some apple cider vinegar or a stronger antifungal and actibacterial, would be a good idea. Iodine soaks are good too. Putting the horse on antibiotics along with giving some probiotics may also help.
At 35, do you think your horse may need some supplements to his/her diet to strengthen the immune system and maybe a biotin supplement? There may be some nutritional extras needed at that age which could be contributing to the hoof problems too, possibly. I'm just throwing out general ideas here for your consideration to add to the practical hoof care you're giving.
I think it's great you're learning to trim your own, and I hope more horse owners are inspired to. Sometimes a person can't get a farrier when needed (or a decent one in the area which is sometimes the case) then they aren't helpless. Even if a person has a regular farrier, I think hoof care tools and knowing how to trim is a good practice.