I have been on Methadone over 20 years im 40 now, and like so many I put on quite a bit of weight. going up to about 250lbs I'm a 5' 8" Male. I tried many diets / healthy eating plans but could never sustain the weight loss.
About two and a half years ago I started looking into a whole foods plant based diet, although I don't like calling it a diet because I still eat this way and its just my lifestyle now.
I'm now down to my ideal weight and where the same jean size i did when i was a teenager lol 150 lbs. I didn't go to the Gym or do crazy intense workouts I walked, I have a step counter on my phone and in the beginning I made sure to get in 6000 steps a day, and after a few months I started feeling so much better I increased that target to 10,000 steps a day.
The key is calorie density plant foods are less calorie dense then animal products and highly processed food so you can eat a larger volume of them, I also use virtually no oil when cooking each tablespoon of oil contains 120 calories, and there like stealth calories you don't notice them they don't fill you up.
All plant foods have some protein so don't worry about where your protein is coming from if your eating enough calories your getting enough protein.
an example of what I eat
In the Morning I have a bowl of oatmeal ill add ground flax seeds, 2 tablespoons of raisins, some cinnamon and this might sound strange to some but I add 3 or 4 tablespoons of beans, red kidney beans or black beans.
Beans are great for keeping you full for hours.
I usually eat fruit during the day apples and bananas.
I have dinner around 4 and another meal around 7.30
Dinner can be something like a bean chili with Rice.
I think it's important to have some starchy foods with your dinner, potatoes, sweet potatoes, Brown Rice Beans, lentils etc.
Keeps you satisfied.
Some tips to make things easier frozen vegetables are a real time saver and there just as healthy.
I also got an electric pressure cooker something like the Instant Pot it's a real time saver.
You can cook dry beans in 35 to 40 mins.
Remove all sources of temptation dump the biscuits, sweets all the junk food because if your anything like I was you somehow find your self eating a packet of biscuits at 11pm or a tub of ice cream.
If your hungry between meals eat something have an apple or a salad and a cup of tea or water, will power can only get you so far this is not a starvation diet.
I found that after 6 weeks I had built new habits and it became routine to eat healthy and get my 10,000 steps a day in.
This worked for me not only did I loose the weight but I have more energy now and feel healthy, another side effect of loosing weight is you don't need as much methadone because its stored in the fat tissues.
There are a ton of healthy recipe ideas out there search for Whole food plant based no oil and you'll be amazed at the options.
Don't be afraid to eat simply eats easier and your taste do adjust.
I hope this helps someone, good luck you can get passed this and its amazing when you do, be patient give it 6 weeks and if it doesn't work you can always go back to what you were eating before, but just give it chance and there's a good shot you will have the same results as I did.