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.