I lost around 13kg over about a year at age 59. I wasn't hugely overweight and I was always pretty active - gym 3 days a week, run Couch to 5k, dance studio 3 days a week, and I walk everywhere. But ... it was food. Comfort food, and my portion control was not good.
For various reasons I changed gyms and got a couple of free PT sessions - I now pay for them (a lot!) but it's so worth it. The first thing the PT said to me: "You can't out-train a bad diet."
For some reason it really clicked.
I counted calories - I'd never done this before because counting calories always caused me to crave food. But for some reason, I got into a type of eating where this didn't happen, this time round - I guess because at 81kg and 166cm I knew I had to do something.
I do a kind of moderated Atkins/keto pattern of eating. I limit white carbs (so nice bread & butter is a treat on the weekend), and I eat a lot of vegetables (but not peas or potatoes very often). I upped my protein.
For a year, I used my FitBit app to map my steps. I found I wasn't as sedentary as I thought - about 12,000 steps per day on average, and at least 2 days a week double that - I've done 25,000 steps without really noticing it.
I put EVERYTHING I ate into the diet bit of the FitBit app. I was accountable to myself. I weighed everything, so I got a sense of what 100grams of cheese or muesli or whatever looked like.
And seeing the numbers really helped me to make better choices - calories vs. satiety vs. nutrition. So 0% fat Greek yoghurt with a teaspoon of honey - really delicious and very filling. Chicken breast - nutritiously dense protein for minimal calories. And as much broccoli, or tomatoes, or peppers, or spinach as I like. I put a tablespoon of mayonnaise on the broccoli, or soy sauce, and it's a quick easy filling meal.
I also try not to let myself get hungry because then I don't make the best choices. So I eat a 50 gramme chunk of cheddar cheese as a snack, for example. Again, gets me protein, is satiating because of the fat/protein combination. And although a chunk of cheese (no biscuit or cracker or bread with it, mind!) is not a conventional snack, why not? Conventional snacks can be bad nutritional choices for you.
The other thing I did was start training in heavy weightlifting. I now lift heavy once a week, and train in general strength and what's now called "metcon" metabolic conditioning - at least another one (sometimes 2) times a week.
But you could start with body weight work: squats, jumping squats, push ups, work towards pull ups, and so on.
It will take time, and you really need to enjoy it! So find a way that doesn't feel like permanent deprivation. Heavy weights help, but don't do it without some supervision or training. And take it slowly. Probably you don't need to take it as slowly as me - but I'm old. It took me about a year to get to a 75kg deadlift.