First, you just have to sweat to work out. Even if you do it in a pool so you don't notice, you HAVE to do it. You need to embrace this fact, and wear your sweat as the badge of honor and effort that it is. You *earn* your sweat.
That said, my motivation comes from:
1) not wanting to feel terrible. I felt so unbelievably bad (basically sick) for the months leading up to my weight loss, and I never, ever want to feel that way again.
2) loving feeling better. I can do things now because I'm more physically fit - like play golf for a full afternoon or even just, say, run up the stairs without feeling winded. I take advantage of that as often as I can and am thankful for that feeling. Through exercise I've managed to become in better shape than a large portion of my skinnier friends. I no longer feel like a prisoner in my own body.
3) Fear. When all else fails I google "diabetes," sometimes "diabetes and blindness" or "diabetes and amputation." It's grizzly. It's also real. Thank God I seem to have narrowly avoided it, at least so far.
4) I don't give myself time to de-motivate. If I'm going to exercise when I get home from work, I do NOT sit down before I start. I do not eat before I start. I immediately get to it and I don't stop until I'm done. If I'm exercising first thing in the morning, it is the FIRST thing I do in the morning. Everything else that I "have" to do comes secondary.
5) I do it for *me*. Not because I "should," not because someone else wants me to. For *me*. Therefore, there is no "cheating." There is only the decision of whether or not to do it. I don't know you specifically, but I do know myself and members of my family who have had (severe) weight problems, and it seems to me that what you're doing mimics what I and they used to do. You're not exercising and you THINK you want to do it. In my opinion and my experience, however, if the pressure to do it was coming from yourself, then there would be no conflict. Finding that reason to do it for yourself is something that no one else can really provide, and it takes a LOT of self-reflection. I've said it before and I'll say it again: the diet's the easy part. The hard part is getting your mind and will to line up with the reality of the lifestyle change.
You've had good success with the diet, though, so you've got to have something in there...figure out what it is and go with it! And if you stick with it for a month or two, you'll start to feel the effects on your health the rest of the day, and then it will snowball. You'll be so glad that your energy and strength have improved that you won't be able to imagine going back. I know that in my case if I were forced to choose between diet and exercise, at this point I'd take the exercise and never lose another pound (luckily I don't have to make that choice, though!

), because its effect on my overall wellbeing is dramatic.
Also, for a regimen that I enjoy for strengthening your back and legs, you might want to look into a Kettlebell. They make lighter ones for women, and they improve strength, flexibility and have good cardio effect as well.
Good luck!
-j.