I’m not naturally slim and up to a few years ago I struggled to lose weight. But turns out the lack of a functioning thyroid was probably a huge factor, I found this out at the same time I was diagnosed as T1 diabetic (yes, adults can develop it).
Being properly medicated for my thyroid and radically changing to a low/lower carb diet caused me to go from 70kg to 52kg (size 14 to size 8) in 6 months. I’m 165cm (so 5ft 4).
I looked horrible at that weight and have gradually put on weight. My ideal weight I think is 58-59kg (size 8/10). This is both a look factor and at this weight I use insulin most effectively. But for my height even 5kgs heavier I’d still have a healthy BMI.
If you’d look at me you’d say I’m slim. There’s also some body “construction” factors. I’ve always, no matter my weight had slim legs and they are also quite long.
I also, prior to losing weight, has quite big boobs - big boobs weigh quite a lot 
I now eat between 30 - 60g of carbs a day. Most of this is vegetables, full fat Greek yoghurt, fruit like berries and kiwi.
I do drink coffee, but also drink quite a bit of herbal tea and water. I also drink wine only. Pretty much red or dry sparkling. And mostly once or twice a week and rarely over 2 glasses. I eat a decent amount of protein and do eat fats.
Exercise - I’m getting back into weight training now. I’ve struggled to manage exercise and good blood sugar management. But I do walk quite a lot.
My advice to people who are struggling to lose weight is to look at carb portions and cut those. Replace “white” with whole grain and halve your portion. Use good fats and protein to fill you up.
Replace that portion with some vegetables. Replace potatoes with another root vege like pumpkin, parsnip or Swede for roasting. Aim to have half your plate be green vegetables. Learn to season and dress vegetables properly, this makes a huge difference to taste.
Get your blood sugar and thyroid checked if you are cutting calories and not seeing results. A malfunctioning thyroid and even slight insulin resistance will sabotage your efforts.
Also look at stress management and sleep. Both of these can contribute to weight gain, I find the body chemistry fascinating and researching it really helps me understand the importance.