Plateau's stink. You did the right thing by changing your diet tho! Your other option would have been to increase your cardio. :-)
In the past when the exercise and diet fell to the wayside, what happened? Did you set a new goal after meeting the first one? Did you push hard to reach that goal, only to be too exhausted to continue when you got there? I think you're totally on the right track about considering it a life long goal. What is it you really want? To be thin? To be healthy? To live forever? Examining your motivations will help guide you to the Things That Work.
Also, not only CAN you do it, you're ALREADY doing it! :-)