You just need to "git gud", as they say in gaming parlance. Sticking with the underpowered side will force you to hone your skills, whereas playing the "easy" side will make you become lazy and complacent. When the game first came out, Union sucked, and I mean sucked big time -- they were useless at everything but running away. You couldn't fire in melee, so as soon as the Rebs touched your brigades you could kiss them good bye. And although their arty was good, no limbering severely limited their usefulness. Playing Union was a big game of pressing the Fall Back button, but it was also a ruthless school of UG:G that instilled sound playing principles through tough love.
Then a couple patches later, Reb became underpowered. Melee was useless and only made you lose condition. An entire generation of players who had built their success on Column Charges (drawing a long line in the general direction of the enemy and pressing the Run button) massively switched to Union. Most openings in MP games were for Reb then, but it was still viable.
The latest patch is the best attempt at balance I've seen so far. Both sides have their strengths and are quite viable. Reb firepower is better, their arty isn't useless, and properly timed charges can be successful; Union morale is better, their firepower is still devastating, but they can hold out in melee long enough to drive back the Rebs if needed.
tl;dr: l2p