When I first read this I wasn't all that impressed. It seems circuitous and unhelpful.
You can be invincible, if you enter into no contest in which it is not in your power to conquer.
That sounds a lot like: "don't try and you can't fail". As usual though, the message is a little more subtle than that.
Take care then when you observe a man honored before others or possessed of great power or highly esteedmed for any reason, not to suppose him happy, and be not carried away by the appearance. For if the nature of the good is in our power, neither envy nor jealousy will have a place in us. But you yourself will not wish to be a general or senator or consul, but a free man; and there is only one way to this, to care not for the things which are not in our power.
Some time-honoured advice here: being successful doens't necessarily make you happy, so don't envy that success. Furthermore, realise that the only success you seek is to be free. Your freedom is gained by focusing your will on things that are in your power and discarding those things which are not. Which brings us back to the opening line: by focusing solely on those things within your power, you give yourself the possbility of perfection.