You are not short on time. You do not fail to stick to your goals because you're too overworked. This is why…
There are lots of different reasons that anyone fails to achieve a goal and to say that any of these is the 'main' reason and more important than the others is a bold claim. That said though, if you define 'main reason' as being one of the main contributory factors – which would most greatly increase your chances of success if it wasn't present – then you can certainly make a strong case for 'lack of belief'.
And by lack of belief I don't mean Disney-style 'believe in yourself' fluff. Rather I mean lack of belief that the program you're following will work. If you don't believe 100% that what you're doing will definitely work, then you are far more likely not to see your goals through.
For example, let's imagine that you've bought a brand new e-book on how to become a billionaire. The book was well-marketed so you were willing to hand over the cash for it, but upon receiving it you find the required time and money investment to be rather high – you need to buy webspace and build a website, you need to write an e-book or hire a writer to do it, you need to send thousands of e-mails, you need to learn about SEO… it's a lot and when you weigh up the options it never seems pressing enough to do those things or give them any kind of priority.
It's the same with getting in shape. You might write yourself a diet plan and a gym routine that you hope will make you into a lean, mean fighting machine, but then when you realise how long you're going to have to go without chocolate cake, or how tired you are when you next need to hit the gym… suddenly sticking to the program seems like an untenable aim.
So does that make you lazy? Does it mean you lack integrity? No: because when you think about some of the impossibly difficult things you do accomplish, it's obvious that you actually do have sticking power when you really need it. You've probably woken up at 7am five days a week every week for the majority of your life for example. You've saved enough cash to buy yourself a home, you've stayed late at work regularly to make sure you progress your career and if you're a parent then you'll have bent over backwards to ensure their happiness and their health.
So what is different between those goals and the ones you don't stick to? Simple: the results are guaranteed.
The problem with the e-book on how to become rich is that you start picking holes in it as soon as you start reading it. Sure a website can make you super-rich, but there's no guarantee that it actually will.
Likewise when it comes to training programs – you know that working out regularly and eating well are both things that will be good for you, but that doesn't mean that you'll be guaranteed to get the body you want. After all, factors like genetics play a role and you know there are going to be times when you can't make it to the gym.
So suddenly, all of that effort might not look like time well spent any more – because there's no guarantee it would actually work. Instead of thinking 'I really should be working on that website', or thinking 'I really should be at the gym', you instead think on some level 'working on that website is really just a waste of time when I could be doing something useful' or 'I'm kidding myself if I think the gym is going to really help'. Now working towards your goal feels like reckless gambling, and as our brains are wired to be risk averse, that doesn't end up boding well for our ability to stick to those goals.
And the worst part of all this? By then not putting effort into achieving your goals you actually end up reinforcing your belief. You only go to the gym a few times, you give up, and then you think 'man I was right, that really didn't work!'.
Now imagine that you knew 100% that a certain workout would definitely give you the ideal physique that you always wanted. Or that two hours a week of work would definitely help you to become a billionaire in two years' time. In those scenarios of course you would then stick to that training program.
(This does only work however if you are actually really motivated by whatever the intended end result is. Working towards a goal you only half care about won't have the same effect even slightly as working towards a goal that you feel would positively change your life for the better.)
So now you know the problem that may well be holding you back from achieving many of your goals… what's the solution?
Unfortunately there's no guaranteed solution, but there are certainly a number of effective things you can do to help improve your chances of success.
One option for instance is to try harder to identify routes to your goals that you feel highly confident will definitely work. This is one of the reasons that P90X and Insanity Workout are so popular: there is a lot of evidence that they work, and they're intense enough that we intuitively know they probably will be at least partly effective (though you needn't spend the kind of money they charge to get an as-good workout).
This is also why people love diets that involve fasting, and why they love things like diet pills – they're extreme enough to get rapid results. The downside is that these methods are dangerous and don't work in the long term, so your job is to do the research and to find the methods that are intense enough to feel definitely effective while also being scientifically sound in the long term and safe.
The next thing you can do is to look at the benefits that you will get even if you don't reach your goals. For example, you may not get abs of steel by working out on a regular basis, but you will certainly be healthier, wake up feeling fresher in the mornings and improve your energy levels. Focus on those things because you know they're definitely achievable.
Finally, work on your own approach to risk and your own levels of optimism. The difference between the people who follow the e-books and become rich and the people who give up before they even start is that the first group are optimistic risk takers. A little more of that would be no bad thing for any of us!