[jmmx3]

I know there's already a ton of threads about overweight yorkies, but I couldn't find anything to answer my questions. So I apologize in advance for being redundent.
So my yorkie is about to be three years old. Originally the breeder told me she would weigh about 4 pounds. When she surpassed that, we thought his estimation was off. But now she weighs almost 8 pounds. When she was at 6 and a half the vet told us she was overweight and that she should weigh about 5 pounds. However, I don't know what I'm doing wrong! We take her on excessive walks, sometimes as far as 4 miles before we have to pick her up in fear of over-excersizing her. She is extremely active, sometimes running off to scratch on neighbor's doors to play with their dogs. She doesn't eat much, probably about half a cup a day. The problem is we usually have to mix something in her food to get her to eat. We've tried stopping, (it's usually a pinch of bacon bits or some broccolli), but she won't eat and will shake because of low blood sugar. She eats beneful dry food, we tried switching her to the weight loss formula but it was basically a laxative and she became unable to control herself inside the house.
I don't know what to do! I don't want her to be unhealthy. She's my first dog and I worry about her! Is it possible she's sick? Or that I'm doing something wrong? Or is it her food?
Sorry this ended up being so long. Any constructive criticism or advice would be totally welcome. Thanks