You can be unconquerable if you only fight battles you can actually win. So when you see someone with high honors, power, or great reputation, don't be fooled by appearances. Don't call them happy. If what's truly good comes from things you control, then there's no reason to be envious or try to compete with them. For yourself, don't try to become a general, senator, or consul. Try to be free instead. The only way to freedom is to stop caring about things you can't control.
You can be unconquerable if you enter into no combat in which it is not in your own power to conquer. When, therefore, you see anyone eminent in honors or power, or in high esteem on any other account, take heed not to be bewildered by appearances and to pronounce him happy; for if the essence of good consists in things within our own power, there will be no room for envy or emulation. But, for your part, do not desire to be a general, or a senator, or a consul, but to be free; and the only way to this is a disregard of things which lie not within our own power.