GM, I can't tell you how many nights I've promised myself that "Tomorrow I will go for a nice walk" or "Tomorrow I will get on the stationary bike for 20 minutes" but I never do it.
You and I are both heavy gals and I really think that once we have all that extra weight on us, our bodies are just plain exhausted trying to support all those extra pounds. It's all I can do to drag myself to the pool two or three times a week and do a fairly vigorous workout in the deep end. Still not vigorous enough though. I should probably be taking an aquafit class where they bounce around a lot in the shallow end. It's just too easy to drift when you float as wonderfully well as I do. LOL
While I'm not making excuses for my basic laziness, I think we really should be kinder to ourselves. Perhaps we shouldn't get our hopes as high as we do for rapid weight loss either. Our bodies didn't get this way because we have supersonic metabolisms. Those fat cells want to hang onto every fat molecule. They're hoarding them!

Perhaps we should set more realistic goals for ourselves. Then we won't be so disappointed when we don't reach them. If you've lost 12 lb since Jan 1, you are doing very well, according to all the diet experts. That works out to 6 lb a month. At that rate, in a year you'll have lost a whopping 72 lb. !! Think about it. I'd be totally thrilled. Also, consider this. Once you've lost another 20 or 30 lb, you're going to have a lot more energy and will probably feel like exercising more so that could even speed up your weight loss a bit. Something to look forward to.