I have said it before but I think willpower is overrated.
I think you have to do the internal and environmental work to make it easier and then the rest follows. I read a book years ago called "influencer" (because that now has a different meaning, I think later editions are called "crucial influence") which I found helpful for this. It helps you in behaviour change for yourself or others.
You can find links to the worksheet which basically sums up the whole book all over the t'internet. Here is one:
six-sources-worksheet.pdf
What you do is look at three different bits of your environment. You (the personal), social (your friends, family, connections, things you attend) and structural (what's in your kitchen, fridge, how you shop etc). Then you look at all three of those from two aspects. How can I use this to motivate me? How can I use it to increase my ability? So it's all about incentivising what you want and removing blockers, making things easy etc.
So, for example for you:
- Personal motivation - why do you want to lose weight? Imagine it. Perhaps it's you looking great in your favourite outfit for a specific event on a specific day.
- Personal ability - do you have the skills to make your 500 cal days work. Do you have the nutritional knowledge? Are you lacking new and interesting recipes?
- Social motivation - do you have a group of people cheering you on, are friends and family on board and helping? Can you have a friend you message when you're finding it tough?
- Social ability - is this something you can do with a friend or get help from someone?
- Structural motivation - how will you reward yourself for doing the right behaviours? (I've explained what I do below) but my first meal after my fasting day is always exactly what I want which is often the same meal. But I look forward to that meal, cooking it, enjoying it, it's actually pretty healthy but it makes me look forward to fasting day. I also incentivise my own weight loss which I'd rather I was incentivising my behaviour but I'm working on it.
- Structural ability - I moved all foods I don't want to eat on any day into drawers in my house so my partner and kid can get them but I don't see it. It's a tiny nudge but I think helps.
I think you need to choose a diet method that you like and works for you. I tried 5/2 and hated it even though I stuck to it but have found once a week 24 hour fasts easier. I know that sounds bonkers but it's really not. Not eating breakfast and lunch for one day basically. But if you've had success with 5/2 before then great, stick with it.
My only watch out though is I don't know your age. I found what worked for me at 35 stopped working at 45. Perimenopause is a bitch for weight gain sadly.
Also, I know this is hard and I'd have hated to read it a couple of years ago. I gave up drinking last year. Lost me f. all weight mind you but gave me more motivation to be healthy. Not saying you need to but just giving my experience.