I have a 2012 cyclocross mares ax 2.0. I was briefly "fitted" for it when I bought it new, but I think it might be too big. It's a size large 56 cm. I am 5'10" but I always get lower back pain if I ride the bike for more than half an hour. And I find the reach quite stretched, so I'm sitting forward on the saddle and my shoulders take my weight and so they get sore too. Any time I look up sizing though, recommendations say I should be on a bike around that size? Do I need a smaller size bike?