First step, know that this is normal. It happens to me all of the time. I've been so happy since recovering from depression but I suddenly have these days where I feel depressed again for no reason. But know this, these attacks of sadness are normal and short. Second step, follow my tip the best you can: take some you time. Don't hang out with that guy that you hate but only talk to because you feel bad that he's alone, don't drink that tea that you hate just because you're on a diet. You need to take some time to comfort yourself until you get out. Hang out with people that make you laugh, read your favorite poems over and over, buy some cheap Pillsbury turnovers and make them in the oven, write a page of all of the achievements you've made, write some memories of one of the best trips you've been on. Reflect on the good stuff, comfort the hurt parts. Excising is like that, you tear up your muscles and it hurts for a few days, but when they grow back they are much stronger than they were before.
Accept these days like a torn muscle, although the pain is almost unbearable, know that when you are feeling this pain you are also welcoming healing in to make you a better person. Improvement is not a line shooting straight up to the top of the graph bar. Improvement is a bumpy road with dips and hills that slowly make it up to the top. Accept that these days will happen, but that they only bring healing and in the end, improvement. These days will come, so let them come geared up with a can of frosting and positive attitude. Don't be tough on yourself. Don't get mad because you feel like you've lost all of your improvement by having these days. You, torn muscle, are on your way to becoming a stronger and more experienced person.
You can do this. I believe in you.