Hi TH - Welcome!
I can relate to a lot of what you said. I struggle with depression and my weight - boy do those two just feed off each other. I've been having good success with what I am doing now, and I think the advice the others have given is spot-on.
When I would go on X diet (whichever one), I would always fail, because like you I was all or nothing. The TINIEST bit off plan would completely blow up everything I was trying to do. What has worked for me is working with my doctor, and making healthy changes to my habits. My doc did a health/weight assessment which told me my resting metabolic rate (RMR), and then we created simple daily calory goals based on it. There are calculators on the web that can estimate your RMR or BMR (basal metabolic rate). This tells you how many calories your body processes each day.
Thats it - there are no no-no foods, no gimmicks, no special formulas. The trick was for me in stopping using food as a pleasure/pain device, and simply acknowledging that food is only one thing - a source of energy for my body, Thats it. It has no other power. Wow - once I really got that making healthy choices became so much easier. Would I fill my car with gas that I knew was filled with oily, disgusting sludge that would hurt its engine? No.
I, like the other posters, count my calories. I write down everything I eat, and subtotal it throughout the day so that I know how many calories I have left in my "budget." This helps me plan for dinners out or fun with friends.
My doc also put me on an anti-depressant, which is helping me with my depression. I couldn't treat one without treating the other.
One last suggestion I would offer is this, although I can't tell from your post if it will be all that useful: let go of your memories of what you were like when you were thin, and all the times you tried and failed to lose weight. Those are not today. Practice compassion for who you are now, and commit to doing the work you need to do today to make yourself healthy and happy. Don't try to do tomorrow's work, or the next days, or think about the sports you used to be able to do. I used to be quite fit and at the time was an avid runner. Because I held on to my internalized memories of that old me, the current me never seemed to measure up. If I'd exercise and get winded walking I'd do a lot of negative self-talk about how worthless I was and how much better I was when I used to be athletic. All this did was create deep hurts that have taken me years to work through. Would I love to be as thin as I once was? Sure. Would being thinner make me the person I was back then? No. That me is gone, and I am trying to be the best me I can right now.
Good luck! You are most certainly in the right place. The people on this board are some of the greatest people in the world.