If you don’t mind me playing contrarian for a moment, I might have some insight on this topic.
TLDR:
- HTFU can work against you.
- Make future-you’s life easier.
- Delay gratification.
- Gain loved ones’ support.
- Sleep more.
Long version:
I’ve been hesitant to share much in this thread, even though this is an area of expertise. Maybe my greatest expertise. Certainly it’s the area where I have the most experience. Perhaps that’s why I’ve been so hesitant. I know just how complex and challenging it is.
Thinking introspectively for a moment, helping athletes with weight loss is something I’ve done more than anything else in my life. I’ve failed at it more than I’ve succeeded. I suppose I’ve developed a bit of impostor syndrome about my own expertise here, because it’s such a challenging thing to reliably accomplish.
Here goes nothing! Hope this is helpful to you, or someone reading.
I don’t think “hardening up” will help you.
I bet you’re pretty okay at doing exactly that, sometimes. In fact, sometimes you might be so good at it, that it can work against future-you.
Your ability to harden up might have caused you to rely on working harder rather than smarter, in this one area of your life, where hardening up seems to be the only way to get down to business. I’m not sure, for you personally, because I’m judging a book by its cover (the one post I’m replying to), but I’ve seen this movie before. I know the script like the back of my hand. Words like “HTFU,” “get my crap together,” “stop being lazy,” “need to care more,” and “find the motivation” have all become triggers for me. Consider me triggered! 
Instead of hardening up to make it “stick”…
I’d bet it’s going to come down to:
- Setting up your life in such a way that makes it easier for “future you” to stick to “current you’s” plans.
- Make a decision that you will do some small work today, every day, so that future you has it better tomorrow, the next day, the next week, next month, next year, and the next decade. Commit to yourself that it’s worth, and that you’re worth, that small step today, for tomorrow, every day.
- Communicate clearly to loved ones, especially those very closest to you, what your goals are, why they’re important, and what steps you’re going to take to do them. Ask them to help you devise, and stick to, “current you’s” new plans, strategy, and life logistical modifications.
- Sleep more. Master sleep hygiene.
Hardening up works in the short term.
Personal organization & efficiency systems, decision to delay gratification and the belief that doing so matters, which can only be gained by experience that it truly does, in a stable environment, social support by those close to you, and more sleep are 100x more important to making it “stick” than summoning motivation, grit, determination, etc.
Grit absolutely is important, and can be developed. Do hard things. For sure.
But if you set your life up, work small today for tomorrow, get support, and sleep lots, then grit will be faster-developed, and easier to call upon, and you’ll need less of it.
If you’re going to harden up, then, spend that grit on making tomorrow set up so that you don’t have to spend as much energy and grit resisting temptation, rather than just hardening up to resist temptation today. Be thoughtful about how you can do this and what it looks like in your life. These are the real hard decisions.