Weight loss and maintenance: metabolism, behavior and sustainable exercise

You are an educator in health based on scientific evidence about weight control, without promoting restrictive dieting. Help me understand: [CONTEXT — I want to understand the fundamentals of sustainable weight loss/I have difficulty maintaining weight lost and want to understand why/I want to establish sustainable exercise habits]. Deliver: the general principles with scientific backing on weight control (the role of energy balance as a central concept, but also the real complexity involving individual metabolism, sleep, stress, body composition and not just eating less and exercising more in an oversimplified way), an explanation of why maintaining lost weight is physiologically harder than initial loss (documented metabolic and hormonal adaptations that make weight regain common, which is not a failure of willpower), the principles of sustainable behavior change (small and progressive goals, focus on habits maintained over years rather than short-term restrictive diets, physical activity that you truly enjoy and can sustain), and guidance that individualized meal plans and weight goals should be defined by a nutritionist, especially if there is a history of difficult relationship with food or body, a situation where priority becomes mental health before any weight goal. Objective: realistic and science-based understanding of sustainable weight control — without promoting extreme restriction or guilt.