It is asymmetrically difficult to lose weight compared to gaining weight. The body wants to maintain homeostasis, but tries way harder when it perceives the threat to be starvation. This makes a lot of sense considering historically that conserving energy rather than expending it would probably have been a massive survival advantage.
As an example, say we are discussing someone whose average energy expenditure is 3000 kcal a day and they eat that much currently. Let’s say they start to exercise and their average expenditure rises to 3200 a day, and they start to diet, so their intake falls to 2500. The body wants to minimize that 700 kcal caloric deficit to maintain homeostasis. The person will lose weight, but by the time they’re down, say, 10 lbs, a normal person 10 lbs lighter than they started would expend maybe 2900 kcal, but they’ll be expending 2800, since their body will start to downregulate processes that expend energy to minimize the deficit. This is why when people are losing weight they might feel cold (thermogenesis has decreased), tired (mental processing has decreased), or any other number of negative effects. It’s the body going into self-defense mode.
Another factor is fat. Sadly, as weight is gained, fat cells grow, and split when they get too big. When weight is lost, they shrink, but they never re-combine. Fat is an endocrine tissue that releases numerous hormones that regulate energy balance, appetite, etc. This means that people who previously had more fat will never (without medical interventions) be “the same” as someone who is the same weight, but never carried extra fat to begin with.
This might sound like “why bother trying when there are so many things stacked against you?” but I couldn’t disagree more. Despite the propaganda masquerading as science that says “diet and exercise don’t work”, they do. Nobody is immune to the laws of thermodynamics - if more energy is consumed than is expended, weight gain results, and if more is expended than is consumed, weight loss results. The main reason all these studies are able to say “diet and exercise are a bust” is because people don’t maintain the lifestyle that reduced their weight to begin with, and usually return to familiar habits.
Luckily, our habits are mutable, and just like our body can play tricks like making us more hungry when losing weight, we can fight back if we know what we are up against. For example, caffeine is a commonly consumed substance that suppresses appetite. Staying busy rather than being idle also suppresses appetite. Not having junk food in your house/office/etc. reduces temptations to help aid healthy decisions. Having healthy food available and prepared makes making healthy decisions easier. Scheduling diet breaks or planning free meals (and calling them free meals, not cheat meals) improve diet adherence.
In my opinion, the way to sustainable weight loss is slight modifications to the environment and behaviors to nudge the system in the desired direction. Our culture is too focused on motivation and discipline and toughing out situations no matter what to achieve our ends. Meanwhile tiny differences compound and do not feel like an immense burden. It’s also worth mentioning the power of a self-fulfilling prophecy: have you ever known anyone who lost weight via diet and exercise, and said it is impossible to lose weight via diet and exercise? On the flip side of the same coin, there are a lot of people who think diet and exercise will not cause weight loss, and they’re disproportionately overweight. Believing something is possible is nearly always a precondition for achieving it. Thoughts become things.
Best of luck on your weight loss journey. You can do it.