"I'll keep gaining weight forever. I won't be able to stop no matter what I do. I will gain and gain until my life hopelessly sucks in the following four thousand ways: 1)....."
Yeah, that's exactly how my brain works too. It will pick some random, seemingly unsolvable problem or fear (fear = false evidence appearing real), and NOT let go. If I'm half asleep (it likes to pull this stuff in the pre-dawn hours) and just going along with the story, I have no recourse. Every bad thing is real, or so it seems.
The trick is to very clearly call it out as a thinking problem, not a food or weight problem. My thinking is causing me pain. My thinking is making me afraid. My thinking is affecting my behavior in negative ways. It's never, "I ate this, so it means...." The meaning is coming from your brain, not your sandwich or your fat cells. Change your brain and you experience the whole world differently. Change the sandwich ingredients, or ban sandwiches, or manipulate your body composition, and nothing has really changed. The same fearful, unquestioned thoughts are driving the experience.
There are probably all kinds of approaches and therapies for getting people to see themselves as separate from their thoughts. Once you get even a little space on The Story of Really Bad Things, you can start to question and rewrite it. The best method I've found is The Work of Byron Katie, the four questions and a turnaround. It's all spelled out here for free:
"I'll keep gaining weight." Is it true? Can I absolutely know that it's true? How do I feel or what happens when I believe that thought? Who would I be without that thought? Turn it around. Is the opposite as true or truer than the original thought? I used to really delve deep into meditating on a situation, feeling it, sitting with it, writing out answers and turnarounds. I filled notebooks. Now, I'll think something stupid, feel that shock of pain, ask "is it true?" and think, "Nah, never mind." I know that if it hurts, I'm telling myself a doozie.
I'm never a powerless victim of fate. In order to keep gaining weight forever and against my will and have my whole life fall apart because of it, I'd have to first believe that I was powerless, that I was at the mercy of mysterious forces. This is tricky, because when we are severely underfed, biology does turn on a kind of emergency override where we are compelled to eat and keep eating so we don't starve to death. We're so extremely hungry, it can feel like we are possessed or powerless to stop eating. The mistake is to think that initial, very necessary drive to eat is "how it is now." As in, "Oh, well. Can't stop eating. See, I knew it." There is a legitimate biological reason we're so hungry and need so much food after periods of restriction and overtraining, but an unquestioned mind will run completely amok with that.
Reality is that when you consistently allow yourself enough food, you do get full. You do get tired of certain foods, and tired of feeling uncomfortable after eating. You do crave vegetables, or look forward to being hungry for a fabulous meal. I like to feel good before, during, and after I eat, so I keep that in mind with all my food choices. If I eat to feel good, eat for pleasure, eat for satisfaction, eat to fuel the happy, active life I want, it rules out the kind of endless, zombie-like inhalation of food that would be required to actually make me gain weight forever. It's not going to happen. I've had to come up with other thoughts to scare myself with at 2am.