Find the exercise that you live for. I started rollerskating this year and I can't stop. Very little on earth makes me happier than putting wheels on my feet and taking off. (Apart from putting blades on my feet which I learned a couple of weeks ago, but we only get ice-rinks at Christmas where I live.) I rink skate, park skate (ramps), inline skate for distance, cross-country skate for winter outdoors. And I'm like a kid at Christmas about it. I wake up in the morning and if it's a skate day, I'm grinning all day. If it's not a skate day, I count down to when it's a skate day.
When life allows, I skate every day, sometimes a couple of times a day. When I'm busier, I still do what I can to get in 3 sessions a week. And when I skate, I usually skate for hours and hours. It would be rare to do less than 2 hours but if I can, I'll skate for 5+ with very little breaks. It doesn't feel like exercise though, it feels like going out playing the best game of my childhood. It's flying.
It is exercise though. One of the best aerobic and anaerobic exercises there is, while also super easy on the joints. I wasn't over-weight when I started exercising but I was 40, out of shape, flabby and untoned with a badly damaged stomach and lumpy c-section pooch. Within 6 months I was lean, muscular all over, had started to form visible abs, had lost a stone. Nearly a year on I'm fitting in size 4 jeans (for context, I'm only 5'1" so not crazy skinny), my lower stomach is nearly completely flat, with only the tiniest pooch (millimetres) on one side over c-section scar tissue. My upper stomach has 4-pack abs. My endurance and general fitness is better than it's ever been in my life. My butt is back where it was when I was 21!!! I'm fitter, happier, better able to handle just about everything.
I never have to talk myself into going exercising. I never look at my body and think I better keep exercising if I want to maintain it. I never have to push myself through a work out telling myself to stick with it for just 15 minutes more. I never reward myself for a good exercise session. Skating is my reward. I literally have to pull myself away from it because other responsibilities are making me stop. I pretty much never get to point where I'm stopping because I don't want to skate or I'm shattered from it. That's not to say I don't work out to near breaking point, I do, I just don't know it until after I've stopped because I'm so happy.
So honestly, find that thing for you and you'll never have to motivate yourself to exercise. It's motivating yourself to stop that will be more of a problem.