One can easily envision Carl going into that moment with the faintest of hopes that it wasn’t a walker bite at all, but some other less agonizing injury, only to face reality head-on.
Surprisingly, it wasn’t immediately gut-wrenching to watch the moment when Carl got bitten, though the haunted look on his face could have done it.
Not the best feeling in the world, and then to immediately follow that with him getting the letter that Michonne wasn’t even around.