Here you can see that there are some things over which we have complete control. There are also things which we don't completely control. This can be split into things we have no control over at all, and things we have some control over but not total. (This is actually a trichotomy derived from the philosopher Epictetus.)
An example of something we have no control over at all is when the sun rises. Things that we have some control over but not total are generally impulses, desires and aversions, so you can control your attitude in a certain situation or what kind of effort you put into a language-learning exercise. Things we have complete control over include our opinions, the goals we set, the values we consider important and our character.
This is a really useful way of splitting things up, and you can maybe see why it might be useful. We should spend no time at all thinking about or worrying about the things that we have no control over at all. In terms of language learning, this might relate to any putative 'innate' or genetically-based learning ability or aptitude. You're stuck with who you are, so there's no point being upset, particularly when it comes to comparing yourself to others. (Plus, a lot of this discussion of 'innate talent' for learning languages misses the point: there are many things that one can do to speed up one's progress.)
The other two categories are really what we should be concerning ourselves with, since this is where we have the ability to influence the outcome. We can set our goals in a useful manner (as described above) and we can change our impulses and perspectives on particular tasks.
The practical realisation from all of this, for a language learner, is that it really makes sense to find ways to set internal goals as opposed to external goals.
Think of a game of tennis, for example: you might have a goal to 'win the match'. This would be an externally facing goal. It is one over which you don't have complete control. Your opponent may have advantages over you that you can't anticipate or prepare for.
Alternatively, an internal goal might be to 'play my best'. This has the advantage of being something you have full control over (i.e. how your effort is directed) and it is not something that can be refuted or quashed by your opponent on the court. Even if you lose the match, by taking this perspective of 'doing one's best' you'll be able to take solace in the fact that there was actually nothing you could have done to change this outcome. Plus, you met your goal (presuming you did actually try your best).
In terms of language-learning, this translates to formulating your goals in terms of process rather than fixed targets. So: rather 'study for 10 minutes every day' than 'reach a B2 level by September'. This way, you're setting yourself up for success and you can focus on what matters in your studies without the feeling of pressure. Same thing with something like writing a PhD. (I've written elsewhere about how you can think of writing a PhD in terms of time rather than wordcounts or sections completed.)
I hope I've convinced you that goal-setting can be an important lever in how you tackle the meta-goals of your language-learning journey. It's certainly been an important realisation for me in how I setup goals and intentions. It also works really well with commitment tools like Beeminder, though that's a topic for another day!
[This is an edited excerpt from my new book, Master Arabic, now available for discounted pre-order here. Use the offer code 'preorderthankyou' at checkout to get 10% off.]