Measure your portions by weighing them, keep a food diary, cut out junk. Have a day off once a week. Add to your food diary how you feel at various points.
I lost weight like this, about 10kg. Sustainably. I worked out the kind of things that I wanted to eat and weighed what I thought was a normal portion. I then served myself 2/3 of this, measured its weight and wrote it down. This is your new portion size. For breakfast, lunch and dinner. For snacks, I ate fruit and veg.
So a typical day goes: 35g breakfast cereal, 125ml milk. Weighed, written in the book.
Lunch: 300g of home made soup, maybe watered down with another 100ml of hot water. One sandwich. Weighed, in the book. Fruit.
Dinner: 130g of cooked pasta, 200g of bolognese sauce. Or the same in chilli and rice, or chicken curry and rice. If eating meat and veg, 200g potatoes equals 130g of pasta or rice.
Eat like this for 6 days a week. On the 7th, party time. But keep your diary and record how you feel. In my case it took me 3 weeks to work out that 6 days a week I woke up feeling great, and 1 day a week I was like death. So I started to question the need for a huge meal and a skinful of beer or a bottle of wine every Friday day off. Guess what, sometimes it's not worth the pain. If it happens to be a birthday, go for it. You are going in with your eyes open, you'll pay the piper and it's worth it. But a boring Friday watching James Bond on your own? Maybe I'll just have 2 beers.
The current maintenance is to do all this but to back off the regime a bit and have more days off. If you start getting off track, then back on the 6 days pw regimen you go.
Oh, and exercise, exercise. But you can't outrun a crap diet, and you can't expect to lose weight without reducing your food intake. You got to where you are by eating what you do, so something has to change.