English show clothes are designed to look elegant, not to be comfortable. You might want to try one size larger than what you have now, so that the coat does not bind. Part of the problem may simply be that English show clothes just don’t have the same freedom that you have when riding in comfortable schooling clothes & no jacket. You might also want to try full seat breeches. I prefer the synthetic suede ones to the real leather ones, as I can run the synthetic suede ones through the washer, and in an emergency, the dryer to real leather full seats. The synthetic suede ones are also less expensive. Either real leather or synthetic suede breeches will help to prevent you sliding in the saddle. Just a word of warning – – you may like them so well that you find yourself buying them for schooling as well and selling all the knee patch breeches.