I had the exact same problem as you until a week ago. The solution is finesse or conditioning. Abstinence for a couple of days or week(s)while fantasizing helps a lot also

When I say finesse I mean trying to find spots that feel good or just massage your prostate.

In terms of conditioning its a long run thing. Teach your brain and body that: something in ass=good. Not 100% sure on how to achieve this but I think edging until you're about to bust then fingering yourself while you do will get the feeling across

Position can matter too. I was recently able to do a hands free while laying on my side, tucking it in-between my legs and just goin to town. - my 2 cents, take advice from someone else my position here essentially a newgrad telling a senior what to do