In the ACW, the Confederates came close to beating the Union army under Grant at Shiloh, prior to Union reinforcements arriving.
Suppose that instead of the Confederates have some initial success, the battle turns out to be a disaster all over.
In OTL, out of around 40 thousand troops, the CSA lost around 11 thousand, while the Union lost around 13 thousand out of around 60 thousand or so.
Assume that Grant mounts a better defense, and has some early warning that he acts upon, and Johnston walks right into him. Result, 25 thousand Confederates lost compared to only say 6 thousand Union losses. Still quite high, but much more in favor of the Union.
Prior to and up to this battle, everything else is the same as OTL, ie the fighting in Virginia is either going for the CSA or is stalemated.