Exercise is the healthiest way to weight loss. Plus, if your already trying to eat healthier, then exercise will speed it up a bit.
In response to your eating habits, you really have to get creative and sometimes, just throw junk out. You have to think of every possible way to make it healthier. I don't eat food from school, I make my own for the fear of what is in the other stuff. Make sure the stuff your eating, though it might SEEM healthy, doesn't have any preservatives, chemicals, fat ect. I've noticed that my problem was what was in the food I ate.
What I do sometimes is every week I go two or three days eating only vegetarian food. That really helped me.
For motivation, I finally just printed out pictures of the thin character I want to cosplay and posted them on the fridge, pantry, bread drawer. So whenever I go there to eat something, I see the pics and am reminded to ea healthy so I can cosplay that character!
Somethings I do on a daily basis to burn some calories (This doesn't include what I do at the gym) are I do 20 situps, 20 push-ups and 20 squats before bed and in the morning. It's quick and easy. With your back problems, I suggest putting a folded blanket under your lower back for the sit ups. Also, if you need to, do girly swim push ups. Cross your ankles, use your knees, push up, go down, lift hands, do another push-up.
To get motivated, I go on health blogs and look at my thinspiration (thin characters I want to cosplay or sayings and cute pics). Getting motivated for the gym is hard. I mean, I'm gonna be doing an intense workout, so I need motivation! I listen to pump up music. I talk in the mirror about how hawt I'll look. I think about all the compliments I WANT to hear.