It is possible, it's just bloody hard. I was properly, badly, heavily overweight, obese actually, for over a decade and I'd tried all sorts until this time just stuck and the weight started to come off.
I truly believe that so much of it is mental - for me at least, I think I had to hit my rock bottom point and stop fibbing to myself. Once I was prepared to be honest with myself, 'I'm really really fat, there is nobody to blame but me, and there is no quick fix', then I was in the right place to do something about it.
I've been calorie counting, changed the way I eat completely (portion control is the biggest single thing for me) and like fluffy123 says, willpower is the most important thing. What has really really worked for me is a food diary - I write down everything I put in my mouth - even if it's a bite of the kids' tea. It's like my conscience. And then I don't let myself go over my daily calorie allowance - if it's gone, it's gone.
However, I do let myself off a night off at the weekend, and have some wine, homemade pizza etc. Or if I've got a night out, I'll swap that night in. I think that's stopped me from giving up, and a blowout these days is nothing like it used to be.
I'm a runner now too, and I find I have to mix the exercise up a bit or it stops having an effect. I've added in weights, and can't recommend the kettlebell enough for a quick and unbelievably effective workout. Also I do 20 min shred, and I've started mixing up my running so I do my long runs at steady pace and I do some short sharp fast runs too. Did my first half marathon this year, which I could never have imagined doing.
I am 5'2 1/2 so a bit shorter than you and I started at just under 15 stone last September (2010). I'm 9 stone 7 today (won't be after Christmas though...).