You can’t control or stop the thoughts directly. It’s your reaction to them that matters. You can chuckle at the absurdity and persistence of the portion police and dismiss those thoughts as automatic looping gibberish. They’re leftover from a time when eating more meant launching a worry sequence and taking it very seriously for hours or days. Now, when my brain spits out a stressful, pointless line of thinking (as it does), my reaction is along the lines of, “Ha! No.” Or, “Good one, Brain.” Literally a 2 second response. I don’t engage and try to sort it all out the way I once did. I know it’s not helpful. The more attention you give unwelcome thoughts, the more they show up. It’s like feeding a raccoon.
When I started backing away from obsessive thoughts about diet and exercise, "Who am I without it?" was too big to ponder. I had nothing. If you've neglected other interests for years, it's not obvious. You can't tick off a list of other things you're excited about or invested in. A place to start is, "What am I curious about?" So, things you might like to learn more about or pursue. If that is still crickets, "What was I excited about as a child?" Reading a book about a new interest, taking a class, or volunteering can help to expand your horizons and give you something else to focus on and feel good about. Then you start to get a greater sense of who you are and what is important to you. Eating as little as possible was a lame hobby, it turns out. Plenty of other pursuits are more fun and make us feel excited and connected instead of isolated and anxious.