I felt that I played the battles fairly well (all victories and a few draws), but by the time I reached Antietam, I was in a similar situation as you. By the end, I had inflicted similar casualties but shattered and only held sharpsburg. My problem was that I realized I physically did not have the ammunition to fend off Union. I am playing on hard difficulty, but the way the odds stack up, I would rather just admit defeat and preserve my army, or fight a scaled down version. In previous battles I absolutely annihilated the Union, but each new battle, they show up with greater numbers and better vetted brigades. It seems simply undoable, unless there is some secret formula for success. I think the game does not reward the CSA enough for a successful early campaign. It feels strangely similar to the realistic outcome tho with a early Confederate success followed by eventual defeat to a greater Union.