Don't think in terms of going on a diet, because that carries the concept of it ending, of coming off it - and going back to all the habits that caused the situation.
First, think of it as a permanent change in how you think:
"I'm going to achieve a healthy, comfortable weight that will protect my health, improve my family's quality of life, lengthen my life expectancy, reduce my likely need for medical treatment, make me feel and look better etc etc. And it's entirely doable."
This will help you feel more positive and excited about it, more in control, more resolute in chasing your goals.
And this will help you change your behaviour:
I made what I have decided are permanent changes, not a diet - I realised I can no longer eat what and the amount I ate before, now I'm in my sixties. I slowly piled on the weight over the last 20 years until I reached my heaviest ever two years ago. I decided a few months back I will not begin 2026 obese, slow, puffing, avoiding mirrors etc.
I'd also avoided the scales - but decided that whatever it read when I got on at the beginning of this would be A Good Thing - because it would be the first of many little steps.
I made a chart / graph in a pretty notebook with my ideal loss of 3lb a week as a line pencilled in, then my actual progress in red. Every weekend (only once a week so as not to get obsessed) I weigh in and put the result on the graph. And yes, I have bought a packet of gold stars and award myself one each weekend there is a loss. I like looking at my graph and my little stars.
I've changed habits, little ones, here and there. I stopped getting the pastry every time I ordered a cappuccino. I don't have three meals a day, and I don't eat late. Alcohol occasionally. Yes, I like fish and chips, cakes, chocolate biscuits - so I might have something at the weekend. Not every day.
Porridge & fruit for breakfast . Dinner is a good piece of protein - fish, eggs, cheese, beans etc, some carbs - potatoes, rice, pasta etc in smaller portions than I used to, and shedloads of vegetables/salads. I bake and grill rather than fry. Fruit for pudding. No random snacking. And no gimmicky and pricey meal replacement nonsense.
I really look at the calorie count on packaging. That cheese triple sandwich I used to get as a snack/meal deal? It's 600+ calories!
I walk as much as possible - now thinking I might like to go to a gym or a dance class as I notice the differences in how I look and feel.
It's not about being obsessed - it's about being mindful and thoughtful about my choices and their implications.
So I'd say - take the first step. Get the pretty notebook and the stars (that's you, that is).
Do the first weigh in. Celebrate whatever it reads as Step One.
Draw the graph (weight in stones and lbs or kgs, whatever speaks most to you , on the left, weekly dates along the bottom and draw in a reasonable weekly little loss goal.
Make all your food choices mindful, bearing all your goals in mind.
Be excited and positive about it and give yourself a star for every single pound down.
It's a journey not a transformation scene. Welcome to the journey.