It's really hard to "convince" someone to want to change. I find that you almost have to hit a sort of psychological wall. You just have to KNOW that you can't feasibly let things get any worse... for your healthy, for your sanity... for your self-esteem. That in itself is often enough to make you want to kick the habit of the junk and start getting down to business.
When I lack motivation, I tend to spend a lot of time on the forums. ****, half the time I'm not even talking about my weight, I'm *****ing about random crap, but simply being surrounded by people who don't think I'm
nuts for obsessing just a little bit about the scale moving, or not moving... it's very motivating to feel normal whilst still trying to promote healthy weight loss.
I think putting together a game plan that's a little more structured might help you in getting started. It's easy to say, "Okay, 12-14 lbs... here we go..." and then feel apathetic and uninterested. Whereas, if you break it down...
So, say you have 6 weeks until your doctor's appointment. Great! So you're aim is to lose about 2 lbs a week, then? Maybe start setting yourself weekly mini-goals, and figure out ways to attain them. Perhaps decide on a certain amount of exercise you're going to get every day, and try to stick to it, day by day. Things are a lot less daunting when you go day by day, instead of just looking at the BIG picture.
Rachie made a good point about mapping things out... a good way to start is to plan out meals and exercise in and around your schedule for the next couple of days, and stick to it! Once you get started, you get more and more motivated.
I also highly suggest checking out the "Chicks Up For a Challenge" board. It's highly motivating, it's a ton of different goals that people are working towards together. There's a Thanksgivings Goal, a Halloween one (or 5..), a Christmas one.. etc. Knowing that there are other people egging you on and wanting to see your ticker go down can be a surprisingly good motivator to drop the junk food, or just say no in the first place.
HANG AROUND LOTS! Being an active part of the forums is probably one of the best motivators you'll ever have!!
GOOD LUCK, and we're here for you!!!