Throughout my life (I'm 43) I've had a fairly consistent theme in my dreams; fighting a foe which is invincible.
Some dreams it's just a mechanical warrior with unlimited energy supply and strength, others it's an army with unlimited resources. In the dreams I fight giving 100% effort and not backing away but there is an underlying fear that I may run out of energy.
I guess the main points are that the enemy is invincible, I am not going to back down, and that my fear is tiring and losing the battle.