I'm sorry to hear that you threw your back out. I've been there. I abused Advil for a few weeks and eventually got better.
As for your feeling of going backwards in your "new life," I believe your fear is unwarranted. Why? Because, life happens. It will continue to happen whether you are on plan, at goal or whatever. The question isn't, "Will this setback derail me from my fitness/health goals?" In fact there shouldn't be a question at all. There have been days when I've been OP and exercising and the scale is going UP. Sure it made me feel upset and hurt that my efforts came to nothing, but did it even once make me want to give up?? Nope. Not once.
The only way you can fail is if you give up. If you keep fighting then the only result will be success. This is indeed a life change. It's not something you're doing simply to get to your goal weight. You can't throw this out the window when you get there anyway. So since you made the decision to get healthy, you can't let an injury throw you off track. Stay the course. Cuz when you start exercising regularly and pushing yourself and whatnot, body pain will be just another part of life. I'm serious.
Keep your chin up dear and have faith in yourself.
P.S. Perhaps you should have fewer trigger foods in the house. If you've got "chocolates/cookies/crisps" in the house, there's a chance you may eat them. And if you eat them, will you be happy about it? Probably not. So since eating them won't make you happy, you should probably purchase alternatives.