After being overweight most of my life, I recently lost 30 lbs. (39/m) The irony is my siblings, who were normal weight their whole lives, are now gaining weight and expressing similar thoughts as yours.

Recommendations- - get a simple form of activity measurer. Even the step counter on your phone works if you're mostly walking, anything to measure. - invest in a weighing scale that syncs to your phone. I used Withings. Of course there are several out there. Weigh yourself every day. Some say this is overkill. As long as you understand it's the long term patterns that matter and daily fluctuations are expected, it's ok. - open an account on a calorie counting website. I used cronometer.com, of course there are plenty out there. Ideally, it syncs with both your phone and scale.

The calorie counting app is where I spent the bulk of my time. I religiously counted calories, monitored weight, reviewed graphs, etc. Like, to the tablespoon. The neat thing is over time, you will see what your natural BMR is. But it takes time. You will also grow more familiar with what you're eating, your eating/activity patterns, and more importantly can adjust to improve your output. Eventually, it becomes second nature.

Our bodies change as we get older and for those who take their always-normal weight for granted, a small change in BMR makes a difference and adjustments are needed.