This is not a good long term fix and I don't know if it will work for you because you said you have long legs but shortening your stirrups might help a bit. Also it's great that you are already doing lots of no stirrup work because that's something that will definitely help!
Think about putting all your weight down through the back of your leg and into your heels. Maybe try shortening your stirrups because it will make it easier to put weight in your heals and then once you can do it move your stirrups back down :)
I have the same problem. The only really helpful thing I can recommend is really focusing on putting weight into your legs instead of your seat which you can't really do with no stirrup work. Also really focus the weight into your heals this should help. And if your finding that you start to bounce badly use your ankles to absorb the shock.