Your diets fail because of stress/emotional eating. That's what you have to learn to stop, it's the only way for you. You can learn how to do that on my website, www.shrinkyourself.com, or search for other sites on the web, or work with a therapist in your community.
I used to weigh 150, and lifted weights about 4 days a week, then I had a baby, and I gained 70 lbs.
I´m now only down to 165, but I too really want to loose weight.
So, I guess what I´m saying is.... you really have to want it, because the first 50 or so lbs I wanted it, and now, well, I just want it but not bad enought to end my eating.
I say it´s kind of like quitting smoking. I haven´t smoked in almost a year, and well, if I had the same mentality about loosing weight, I would have already been back to 150
Good Luck, and only loose it if you want to. I think a size 12 is fine. I´m sure you look good :)
I have also been relying on my faith which keeps me afloat..talking to God (as if He were right in front of me) .....hope it helps