The Gospel and self-control
So, I've claimed that a Christian's job isn't to be self-controlled (or do any particular work), but to believe in Jesus. But I'm also claiming that believing in Jesus will improve your self-control.
Why do we lack self-control? Why do we eat too much, or drink too much, or gossip, or act-out sexually. Is it really because we literally cannot control ourselves? I don't think so. Think of your typical instance of lacking self-control – if someone were there with a gun to your head, you could probably refrain from whatever action.
The real reason we don't control ourselves is that we don't really believe it's in our best interest. We believe (trust, have faith) that our happiness will actually be best accomplished by gratifying ourselves. We don't believe our happiness is best served by Jesus – in short, when we act badly, it's not really so much that we can't control ourselves as that we don't want to. And we don't want to because we don't believe in our hearts that it's good for us.
But believing in Jesus entails believing that He is our shepherd, and under his care we shall not be in want. We indulge ourselves to satisfy a felt need, but trusting in Jesus convinces us that our felt need will best be met by Him.
Imagine you are hungry. You're told by your friend that he's taking you to the best restaurant in town so you can eat. On the way, you stop at 7-11. You have the opportunity to eat a large candy bar. Do you do it? It largely depends on whether you trust that your friend is really taking you to the best restaurant in town. If he is, then you'll control your appetite for a little while longer; if you don't trust your friend, you'll satisfy yourself with a Snickers.
It's the same with Jesus. If we trust Him, we'll control ourselves. If we're failing to control ourselves, the solution isn't to try harder. It's to know Jesus better.