Well, your OP did say "in the shower," which plenty of us have experienced chafing that we don't notice until the shower. That kind isn't near as bad as what you describe...

Trust me, you're not the only one who chafes that bad. All the years I raced my nether parts were indistinguishable from hamburger. But if it's that bad, IMO it's almost definitely saddle fit. It's just, in the 1980s, who knew about that stuff? And, cut-outs, which I definitely need, were only invented right around the time I burned out on racing.

There's still a lot of trial and error involved unfortunately, but reading through the saddle fit threads will let you eliminate a lot of saddles right off the bat and possibly zero in on a few that seem likely candidates.