How do I stick to a weight-loss regimen?

I'm a 22 year old male, 6'1", and about 298 lbs. Obviously I'm a bit overweight for my age and height, and I've wanted (and needed) to lose weight for quite some time now. My family has a high risk for arthritis, high blood pressure, and diabetes and I'm afraid that if I don't lose weight, I'll be head down on or more of those roads soon. My problem is motivation and consistency. I cannot under any cirstances keep myself motivated to stick to any type of workout and diet plan to lose weight. I guess I'm unable to see the long term benefits. I've even tried negative ways to force myself to stick to it, but I don't last for more than about a week or so (which is really week I know.) I guess it also doesn't help that I have unrealistic expectations. I somehow expect to work out for a week and drop about 70 lbs! I know that's not going to happen, but realistically over the next year or two, I'd like to try to lose at least 70 lbs. Any ideas on how to stick to a diet and workout plan? I'm open to almost anything!