Yet you know you've got to do something...so what should you do?
Experiment. Which means that you should try a little bit of everything. I've written on how in our house we regularly spend time lifting weights, running, doing yoga, kicking metaphorical butt with Muay Thai, and lately (at least for Dr. A) joining some Barre classes.
My point is simple. One exercise isn't going to cut it. And doing one form of workout alone is often so monotonous and draining that loses effectiveness and your attention. So you quit. No matter of willpower is good enough to overcome that. And I'd rather you not be bored to death anyways.
Then what's the solution? What exercise is right for you? Any of them and all of them. Try them and see what you like and DO that. Then, try something else. There's no one way that is THE answer here. They all work your body differently, which provides a huge benefit by itself keeping your muscles and mind active. A pro quarterback doesn't just throw passes all day. He runs, lifts, and does other exercise amidst his regular practice. It's the combo that gives most benefit.
A good week for me exercising (one where I feel both excited, engaged, and energetic) includes 3 days of lifting, a day of yoga, and at least one day of Muay Thai. Each one works me in different ways. And I love it. If you're struggling to find a routine, workout, or form of exercise that works for you, maybe the answer isn't finding the "right" exercise. Most likely it's finding the "right" mixture of different forms of exercise.