First things first, I would DEFINITELY cut down on the treats. If she is seriously overweight, I wouldn't even give her treats - they don't "NEED" them. If it bothers you and you DO want to give her treats, I would suggest a low calorie one, such as Charlie Bears (they are only 3 calories per treat) and only give her one per day or one every other day. You can also substitute veggies and fruit for treats - carrots and apples are a great choice. Also, she does need plenty of exercise. Are you able to walk her at least 2 or 3 times a day?
As for food, I would switch because the protein is quite high in that food, IMO, for a Yorkie. It's a wonderful food no doubt, but if it were me I would stick to feeding it to a larger breed dog. Both of my babies are on NV Raw and it is wonderful but it's understandable if you can't afford it. What about Wellness Core Reduced Fat Formula? Although, this food isn't "cheap" either.
Are you free feeding? If you are, I would suggest stopping that. Feed them twice per day - morning and night. If she is severely overweight and you want to help get her weight down quicker, you can start feeding her the amount of food that a dog her size SHOULD weigh. For example, if she weighs 7 pounds and SHOULD weigh 5, start giving her the amount of food that a 5 lb dog should eat. Talk it over with your vet and find out exactly how overweight she is and what she SHOULD weigh. I'm sure he will have some helpful advice to give you
Good luck!