Persistence
Many important things that you can achieve in life require persistence over days, months, years. Such are making your body fit by exercising, making yourself healthy by eating properly, studying a foreign language, developing your business, or changing yourself through personal development.
One area is generally alright for people who work. Many of us eventually become professionals in some narrow field. Our life forces us to work in the same area every day, so we achieve some results. But then why so hard to be persistent in the other areas when nothing forces us?
I think it is because you need to keep doing something that requires persistence for a long time before you see *any* results. If you treat something (for example going to the gym or studying a language) as strictly goal-oriented (you want to see results) then you will quickly see your desires go down.
My solution to this is to avoid being goal-oriented. Instead, you simply decide to change your *lifestyle*. You decide that you will go to the gym because you would like to be a person who does that, you study a language because you like it, or you read books because they are interesting.
Then everything just falls into its own place. You may not see how your body changes after you go for a run a single time. But after you maintain your new lifestyle for some time, then if you do fall back, you will feel that are not the person who you want to be, and there will be something to bring you back on track - your new you. And then, after a few months you might even notice that you run faster or that you are becoming more fit. Or, after you read a few books, you will get more interested in something else. Or after you already don't remember how long have you been getting through the Spanish grammar, you might eventually be able to read una revista en español :)
The results are not the goal, they are by-product. The goal is new you.