I don't see the issue here, as cav you should be doing your best to stay out of range and/or sight of archers/xbows, seems to work fine for me even on my Destrier which is inherently slower than your Courser. Sure you can't expect that you're going to dodge everything, but neither can anyone on foot, atleast you can get out of range far quicker.
Plus if you're getting dehorsed in a big group where you slide across the ground right into someones feet so he can hit you twice, most of the time it's your own error, when it happens to me it's usually because I am just charging in mindlessly, if I play properly usually when I get dehorsed I don't get hit at all or at most I get hit once, then I am fighting on foot.
