your description suggests that you’ve never dedicated yourself to losing fat for more than a 3 week stretch, and that you found that 3 weeks to be ‘mind numbingly difficult’. That’s a big problem. Saying that you’ve ‘given several honest attempts at losing weight’ is simply not true if you have never dedicated yourself to the process for more than a few weeks. If at all it took was 3 weeks to reshape your body, everybody would do it.
Another observation: you likely didn’t gain 5 lbs back in 2 days. You’re almost certainly discounting the degree of water weight fluctuation in the human body. My own body fluctuates about 8-10 lbs every single day. I generally wake up about 4 lbs lighter than when I go to sleep. So I think your observations may be problematic in and of themselves.
So here’s the deal: you need to make good food choices. Calorie deficits are important for shedding lbs, but the absolute best thing you can do for yourself is to learn to cook, and to use quality, fresh ingredients. If your primary sources of calories are vegetables and meats, and you eliminate processed sugar completely from your diet (this includes things like juice. water should be your only beverage), then you will have success pretty quickly. When people cook for themselves each and every meal, they tend to develop much better eating habits, and they tend to eat less naturally, because of the effort required.
And speaking of effort… I see this as your biggest hurdle. You don’t seem to know what real work is. This is going to be difficult, but if you care about your health, you’ve just gotta suck it up for a bit until it gets easier. And it will, once you’ve developed good habits. But it can take a full month or 2 to really ingrain those habits and feel good about what you’re doing.
I hope you can find this dedication within yourself, for your sake. nobody can do it for you.
oh and lastly, do whatever you want in the gym. no program will substantially effect weight loss directly. Gaining muscle mass over time will help your metabolism, but just doing lots of work in the gym is largely ineffective, and it’s likely to beat you down more than help you. Just pick a program you know you can stick to.