You get back on track by putting on your shoes, lacing up, and making your next choice a better one. One choice at a time.
We all slip up and have setbacks. That's ok. You didn't gain 20lbs back with an off week or two, as that would be impossible. Even if you've gained a couple of pounds back, so what? You'd still be down 17 (or so) lbs, which is awesome.
Put your sneakers on and go for a walk, go for the apple, and be kind to yourself.