We can keep this fairly simple by stating that people require energy whether there is a caloric deficit or not. You are not unusual that you have no "off" switch when you have eaten enough. That will change, so to keep it really simple, count carbs and limit them. I know few people who can overeat on fat and protein. As for constipation, that may be a separate thread, and we've have many discussions about that on this forum. Make sure you're getting enough salt, magnesium, and other electrolytes on a daily basis. My final comment is that transitioning into fat burning mode does not require a reduction in calories. It should not result in constipation unless you're limiting yourself in other ways. You may have short periods in the beginning where you have these problems, hunger, constipation, flu-like symptoms, but I assure you they can be managed and will go away. This is a big change in the beginning, as it's hardly trivial to change the energy source priority for your metabolism, but it can be done and you'll flourish in the long term.