Changing those negative thoughts and judgments into realistic and reasonable ones is going to feel uncomfortable and unnatural. In fact, feeling uncomfortable is probably a good sign that this is exactly what you need to be doing to get past your problems.
So when you're unsure about what the problem is, your best bet is to do what doesn’t come naturally. When you find yourself arriving at a negative conclusion about you or your situation, stop thinking that and start thinking the exact opposite. If you’re thinking that there’s something fundamentally wrong with you, tell yourself the problem is in the situation—not in you—and look for ways to change the situation. If you think you’re "doomed to be fat forever," tell yourself that success is unavoidable if you want it; if you’re feeling like a "lazy slug," tell yourself that your “true self” really does want to exercise. You get the idea.
No matter how big, bad or scary the problem seems, you're always just one thought away from turning it into an opportunity for change, growth and progress. All you have to do is find that thought.