Your body is designed to try and keep you alive. It order to do so, it has a complex reaction to food. Hoarding is one of its key goals. Efficient energy production is another. This interplay creates a fascinating battle within. What ever you eat, your body attempts to create the energy you need with the bare minimum food consumed. It then tries to store the rest. Who knows what crazy idea may enter your mind. You may just stop eating. Worse, you may fail to hunt down that deer. This would cause a famine for several days. It is better to be safe, your body tells itself.
Of course, most of us have improved access to food nowadays. We are more likely to be following some fancy diet than to be wanting. As an outcome, ironically, your body uses its default of energy conservation and hoards food if you are feasting. Or even eating the same things everyday. Even if you are in calorie deficit meaning you are eating less than your normal calorific requirement. Think of it as your body going to sleep when things are normal.
To overcome this, you need to mix your consumption of food. On a day when you have a heavy exercise routine, you may want to eat predominantly proteins and eat a reduced carb meal. On a rest day, you may want to eat more carbs. On another, you may want to try intermittent fasting. Or eat a fat rich diet one day. This confuses your body and it is more likely to let go of its storehouse. This is the stubborn fat that you are trying to get rid of.
For a large part of its history, humanity has suffered from shortages. Your body adapted accordingly. As your normal changes, your eating habits need to change with it. And you thought this was easy?