I would say the bike is not far off your size. This is based on the view of you with a flat back. Were the bike too small in a significant way, you would look squeezed and have a rounded back. If your bike is in fact a bit small, pro tour riders use a bike, reportedly, a bit on the small side and can find comfort on long rides. I would try rotating the bars up to lengthen the reach a bit and since this does not cost anything, I would try it. Also worth trying is to move the saddle back slightly, maybe a centimeter.
You are doing long rides where if any part of fit is even slightly off, small discomforts will be magnified so tweak your bike fit in tiny increments as you search for better comfort. I mark up my bike with either a felt tip marker or whiteout pen to make changes while out on a ride. If the change turns out not to be an improvement, It is simple to get back to the earlier position. Over time, bike fit and comfort gets good.