No stirrup work is supposed to be extremely difficult and is EXACTLY what I'd tell you to do. You shouldn't have to sit his trot without stirrups, post like you normally would. Yes it's hard, yes it will hurt, yes you won't be able to walk right for 2 days afterwards...but it will tremendously help your riding. Start slow, but be consistent about it. Do a lap or two each ride at a posting trot with no stirrups and gradually build up your strength, no stirrup canter work will also help. When you get better you can take the stirrups completely off your saddle. (My trainer used to hide every pair of stirrups and leathers in the barn for MONTHS at a time, and we still took our lessons as normal, 4ft courses on large strided hard to sit horses and all (well not me, I was never good enough to do 4ft courses period, but some of the other riders were)).