Just try to give your best for each side, with decisions based on what is visible on the map and in the intel, not on what you know about the global situation and the orders you have given when playing the other side.
This of course demands mental discipline, honesty and moral restraint.
The hardest part is accepting "unfavourable results".
Some people openly favour one side and may "cheat" to obtain better results for their side - that is ok if you enjoy such a game and you are not hurting anyone.
Others try hard to be unbiased and balanced - only to face some battle result that is hard to accept, and they discover that they unconsciously favour the side who just lost the battle
Maybe h2h is best seen as a learning experience which allows to see how things turn out, to go back a save and change orders to see if things turn out differently, or to even avoid the situation entirely if the odds are not to your liking. Then continue with the result that suit you best.
It's all about the journey, isn't it?