I am not quite sure if you are asking for advice or similar experience. It sounds like the ramp is too much slope and not enough support though I would think fatigue would be the culprit more than insertion. Kneeling in front of the couch and using the couch to support her weight might help. For me one of the best positions has always been on my side with one leg over his shoulder. This article may help too Queen Size Sex tips If those suggestions don't help can you give us a bit more context?