1. The easiest way to keep from overeating is serve yourself smaller portions, then don't accept seconds. You need more protein and fiber to fill you up and keep you feeling fuller longer. When you fix your plate, half should be veggies, 1/4 should be protein (meat, chicken, fish, etc) and 1/4 should be grains, such as rice, whole grain pasta, etc.
2. Since you have a bad back, you should talk to your doctor, first, but I'd recommend trying yoga.