Have you tried doing a protein treatment to see if that helps? Also keeping the conditioner off your scalp and just on the length may help too. There are a couple of great articles from Shelli from Hairscapades on how she cut down her shedding, with the biggest help being from protein treatments. The articles are mentioned in this post:

Co-washing and thinning hair?

I know it can be very disturbing seeing all that hair come out in the shower, if it really bothers I would also consider getting checked by your doctor for anything that could cause hair loss and look at any stress factors, health reasons that could be causing it. (I've also seen others mention that suave caused them to shed more than other conditioners, so maybe changing your cowash conditioner could help.)