Question for Fundamentalist Christians?

I am a Christian and was how others balance their careers and keep ambition out of the picture. Obviously you have to have some sort of ambition to get promoted (I am in the Marine Corps and they design the promotion system to be competitive). They tailor you to be selfish to get promoted. But I don't want to have lustly selfish ambition, for living in the flesh is not of God but of the prince of this world. This causes jealousy/ strife etc. We are supposed to be content to just have food and clothing, but getting promoted requires making yourself look better than others. Obviously there is a delimma in every part of the Christian walk, but I am stuck mentally. Jesus said (and I truly believe HIM) we must lose our lives to save it. How do you balance the Christian walk with the sinful wordly broken system??? I've been in the Marine Corps nearly 7 years and fear God, I would rather give up my career than to displease God. But I have a wife+3 kids. Help pls.