First off have you had her thyroid checked? One of the symptoms is being overweight.
My girl had to lose 18 pounds, but because of her arthritis and degenerative knee condition she is allowed NO exercise, so she lost the weight SOLELY on diet.
First off, my suggestion is dump any weight loss food you have it's garbage. Put her on a good healthy kibble (check out
Dog Food Analysis - Reviews of kibble to find a good kibble a 4, 5, or 6 star food). Not sure what you feed though. Anyway, feed her via the feeding guideline based on what she SHOULD weigh not what she DOES weigh. Then reduce that by 1/3 cup per day and replace that with 1/3 cup either canned no salt added or frozen green beans (they're filling but aren't a lot of calories). IMO until she gets down to a decent weight cut out all treats, scraps, etc as well. If you want to add a bit of boiled chicken breast (like 1/8 of a cup) that's okay but I wouldn't do that everyday. And just wait. This is what I did along with putting my girl on thyroid medication and she lost the 18 pounds in 6 months on diet alone and has kept it off for over 2 years now. She's now 7-8 years old so that would've put her at about the same age as your girl when we had the same problem. My girl with zero exercise eats around 940 calories per day (2 cups of Natural Balance Fish and Sweet Potato) and is 61 pounds (where she should be) now and very healthy in the weight aspect of things.
Good luck!