The first thing I’ll say is that you shouldn’t beat yourself up. Even 20 hours is great for you and better than most of the population manages.
Focus on what you can do, and then try to increase it. It sounds like you have a lot of changes and stress happening right now, which as we know, makes many people turn to food for relief.
Some suggestions to help you in the mean time though, first maybe try going keto for the couple days before a fast so that your body is already in fat burning mode and you don’t have the horrible hunger pangs associated with that glucose purge/carb withdrawal. Also, I know how you feel on feeding your kid, I have a wife and 2 daughters, and I’m the cook of the house, so I’ve been cooking 2 meals a day for them while fasting. If it’s something that is a big struggle, prep multiple meals on your eating days if you have the time so that you can have something made up when you need to feed them.
In the mean time, do what you can, don’t be hard on yourself, as that cycle just makes it harder, focus on what you DID accomplish. I also like to look at progress pics on here and other places for inspiration, and watch videos on the benefits of fasting, like Dr. Fung’s when I’m struggling. Once your life gets a little less stressful after your thesis is done, you’ll find it gets easier.