I have the same problem. My hair is definitely thinner. Especially at the temples and behind the ears. I had very thick hair before and it still looks fine when I wear a wash & go but when I put my hair in a ponytail you can see my scalp through the hair at the sides!!
Scary...
I should stop cowashing for a while. My problem is that I have VERY dry hair and it starts breaking when I don't condition enough. So I don't know what the best solution would be.