Didn’t the CSA win at Antietam?
No.
Antietam was a draw.
The sad part was the McClellan had (IIRC) a entire corps that had seen no action and all of Lee’s troops had seen action. A determined attack by the Union could have damaged Lee’s army so badly that it would have been incapable of further offensive operations.
I hesitate to say that the war would have ended with a year because the CSA still fielded armies elsewhere in the South - but the gutting of the Army of Northern Virginia and possible caputre of Richmond by McClellan would have been hard on the remainder of the South.