First day!

That losing weight was going to be much easier than I'd imagined (during decades of overweight/obesity), as long as I made it a point to seek out sustainable, relatively happy (or at least tolerable/practical) tactics that I could turn into routine habits.

That I'd get improvements in a mysterious sense of well-being (maybe from reduced systemic stress?) in addition to more obvious benefits like normal blood pressure, less joint pain, normal cholesterol/triglycerides, etc.

That I didn't have a "slow metabolism" despite being older and severely hypothyroid. I had a problem with portion sizes and too many calorie-dense treat foods.

First, you should not be looking at this time as a "phase" which you enter for a brief time, reach an objective, then go away from. Don't go on a "diet" as it's commonly used in media, as a short-term period of frenzied changes and activity which you will eventually stop when you reach your goal weight. Instead, go on a "diet" as the term is meant to defined: a plan of eating to sustain health. You are just entering a new stage in life; from childhood to adolescence to food-ignorant adult to the new, educated adult you are becoming. The changes you make now should be ones you can sustain for the rest of your life.

Second, human nature resists change. Habits and reflexes dominate much of what we do. You don't think about how you walk; you just walk. You haven't thought about how you ate; you just ate. Changing either of these habits takes time and effort...studies show it takes on average 3 months of consciously maintaining a new behavior for it to become ingrained enough for it to become a habit which you can continue without constantly thinking about it. So forgive yourself for slips as you adjust to your new routine; you are not a failure, not a bad person, you are simply on the road towards change. When you started walking as a young child you fell often; people never called you a failure or a bad person. They simply encouraged you to try again, and again, and again, as often as needed until it became a skill you'd mastered. Well, WE are the community who will encourage you to try again, and again, and again, as often as needed.

Third, change is far easier (for me at least) if broken down into small steps. I didn't go from drinking 2 liters of soda per day to 2 12-oz cans per week in a single step. I started by swapping a single glass for something else, once per day. Once that was habit, I made another small swap, and so on. Since we're in this for the long haul, if it takes 3 years to completely break an old habit, then it takes 3 years. That time will be (hopefully) dwarfed by the remaining years of your life.