I was thinking you might be a
Jousting horse
I don't know when will it end,
this battle I cannot comprehend.
I can't imagine a horse would understand jousting, it's not a normal sort of battle
Now I am helpless in the enemies thrall,
forced to watch my allies fall.
If the rider gets a solid hit from his opponent, he'll be unhorsed; the horse cannot help his rider
I know I am not as capable,
since I have a one-track mind.
Unlike in a real cavalry battle, a joust requires horse and rider to follow a specific, straight-line path
But I am driven and loyal,
equipped with unlimited potential.
Horses are loyal, intelligent creatures, and they do have the potential to win the jousting match given enough tries
Somehow I know I'll be given a chance,
to be once again under your command.
The rider will only win a match if he has good command of his mount
I will reach deep within the enemy's end,
This time for sure I shall transcend!
If the horse gets close enough to the opponent (enemy), and the rider gets his lance solidly in a chink of his opponent's armor, he will unhorse the opponent and win the match!
Hint 1: I only get side-tracked when I meet an enemy
The horse will only deviate from his straight path if the force of the opponent's lance blow is great enough to push him off track