I found your question so interesting because I was just listening to a podcast today that applies to your question, especially the bit where you wrote... "I am the unfortunate type of person who needs extreme diets to achieve any success".
What I was listening to today was a talk about how our beliefs lead to our attitudes, which lead us to take the actions we take, which cause the results we get, which result in the lifestyle we have. This guy specifically mentioned diets, and how a lot of people make the mistake of trying to change their weight by focusing on the action step, which in most cases is a change of diet or exercise. He said this will never work, because as soon as we stop that action we're still left with the OLD beliefs and attitudes that got us where we were in the first place, which will then bring around the same old actions, results and lifestyle as soon as we stop the diet.
I wonder if you'd be open to thinking about it this way... If you knew that your beliefs and attitudes were causing all the actions, results and lifestyle you have, would you be open to changing your beliefs and attitudes and let the rest flow on from that? It's a big shift, but it's worked for me in the past in this way... When I truly decided I wanted to weight a lot less than I did at the time, and I mean TRULY decided, that was a big day for me. Because it meant facing up to the fact that my future, thin self wouldn't be able to munch on chocolate every day, and she wouldn't be able to drink a lot of alcohol... basically I had to make an honest decision and be honest with myself about choosing what I wanted, and what was more important to me. To be thinner, or to eat the foods I love so much. Because at the end of the day you just can't have both. Sure you can have a treat now and then, but you ask anyone who's at a really healthy weight how much of those foods they eat and they'll say not much. So once you TRULY decide, the rest will flow on. If you're tempted to pick up some chocolate, just check in with that thinner woman you want to be and ask if that's the sort of food she eats. Ask her what she thinks about it. For sure she won't want you to pick up and eat that food. Does it make sense how the actions flow out of our beliefs and attitudes?
I hope this helps. I didn't want it to sound like a rant, but it's such a deep topic with me.
Best of luck! I honestly hope you have huge success in the changes you want!