When you yo yo diet your body tears down lean muscle tissue for fuel. By doing this your metobolism slows. After years of dieting your body's metobolism is so sluggish that when you go on a diet nothing happens. It is business as usual for you body by this time because it is used to being starved.

The only way to kick start this metobolism back up is to work out. Build back the muscle tissue you lost. There is no other way around it.