What you describe in the fourth paragraph is precisely what I have been experiencing I say I lack focus. Thank you for putting into words what I could not 
I too think a great deal of the issue can be traced back to poor planning and lack of direction-- I have tried in the past to plan a meticulous practice schedule and while it works short term, I tend to fail in the long term. One of the problems I remember is that I would eventually make unreasonable goals for myself-- they were often too ambitious and had a high rate for failure. This leads to the question-- what exactly do we mean when we say the word "reasonable?" Musically reasonable? Technically reasonable? Is our perception different from the reality?
Would you be able to give a specific example of what a good practice goal would look like? I know it is subjective and different levels of detail work for different individuals, but finding out how you (and others) on this forum approach the problem would probably be a good starting point, I believe.
Depends on what you mean by "practice goal". I don't have goals like "being able to play those 10 bars without errors at the speed of 102 before this day is over". What if I have to interrupt before I have achieved that, will that be considered a failure, or? Should I get punished and disappointed with myself? No, I don't want these feelings. So I would like to say "plan" instead of "goal". I plan to work with certain things, not to achieve them today or tomorrow the latest.
A technical goal like "practice even, punctuated and staccato scales through the whole circle of fifths" are also less tempting to me. I mean, I used to do that. Usually I got tired of it all before having gone through less than half of that goal, and then I left the piano without having worked even one minute with the pieces I was to learn ... and all I achieved was establishing the idea that piano practicing is boring and that I lacked talent, because this took me nowhere. You might think that these scale exercises will make your fingers fly at the end so that you can play like Lisitsa or something. Well ...
What you need is context because without it you will lose your focus, just as we now have concluded, and without focus you might just as well go and do something else.
I think I can give you a direct example of more focused "goals", because some weeks ago I came home from the best piano lessons of my life. I am a rather crappy amateur, 49 years old, my aim with piano playing is simply to enjoy one of my big passions in life. I played in my teens, then did not play for decades, and then started again about five years ago.
But, I happen to know a concert pianist, who in my opinion is the best pianist I have ever heard. (I have heard many ...) Taste may vary, but this is what I think and I am no the only one, by the way. So I highly admire his technique and I got the chance to be in his master class because I dared, as a friend, to ask to join a session which recently was offered. (We had no audience during my individual lessons, thank God for that.) The other students were terrific diploma students, probably some big names of tomorrow ... I listened to their lessons as well. So it was a wonderful opportunity for me, a mediocre amateur, to get lessons on Beethoven from someone who is easily on the same level as Barenboim. And what I just wrote to you about pinpointing the problem AND the solution is what I learned from there, so I will not pretend this wisdom is my own ...
I complained about my Pathétique I which I cannot master at all, but I'm trying to learn it. When the allegro part starts, I fumble around. The staccato chords in my right hand are a disaster when I try to speed up ... after a while I kind of lose control, my fingers start to "scratch" like a nervous chicken looking for grains. At least that is how it feels. And sometimes I cannot fully synchronize my hands. I fumble, I stumble, I fall. So, there is a lot to solve there.
I know that most people would just tell me something like "practice very slooowly and then gradually speed up. And try to relax while you do it, OK?" And then I would sit with this "goal" for a while and hopefully, one day in the future, play more secure and less jerky ... after ten million attempts in different tempi, right?
This might sound good enough, but actually that is not a very good practice goal because it does not really address the problems, as it does not totally IDENTIFY the problems. Note, the problemS. There are several of them here, and the first thing this teacher taught me was the importance of working with them
one at a time. Not try to find the right keys and get the synchronization and the tempo and the staccato accents and the general musical expression right altogether, by just practicing the same bars 22 times in slow tempo and later on speed it up, while I try to fix just about everything on the go ...
So the first thing I have to do is to position my right hand correctly for every chord. Not to play the chords, just put my hand on the keys and check that the wrist and arm position is such that the fingers are in parallel with the keys, then move to the next position by letting the wrist take the lead - not an exaggerated movement, just let the wrist move a few millimetres ahead and let the arm follow, so that I don't try to shift between the chords just by stretching my fingers out without moving my arm - it is easy to fall into that trap when you are in a hurry ...
So, position, position, position, without actually pressing any keys. (If you don't have access to the score of the Pathétique sonata this explanation will be rather confusing, sorry about that.) And in every move also check that I don't get tense, which I should not do if I get dead sure on every position.
That was the first exercise to do, that can be one goal to plan for a practice session, to do the positions correctly. Then the staccato thing - my teacher explained that these quick chords must be played as "wrist staccatos" where you kind of flap your hand up and down from your wrist. Then I got an exercise to learn this wrist staccato, because if you to it in an uncontrolled manner you will look like a spastic: a simple C Major scale up and down, with just ONE finger at a time and a careful observation of the wrist movement every time, and then really check that I stop the downward movement in the right moment, and then also check that my finger lands in a correct curved position ...
As a contrast to the wrist staccato I can also experiment with playing with a movement "from the knuckles" just to notice the difference.
Well, this was just a little example on details we discussed for a while during a lessons and now I work with these details for this piece. The idea of "starting slow and then speed up" was not even mentioned here. So, I got the real problem sorted out, and then I got exercises to solve them, one at a time. It was also easy for me to apply what I learned on other situations and similar problems. The staccato exercise I got made perfect sense when I got it in a context, so now I can work with this boring scale and know exactly WHY I do it - therefore I will not lose my focus.
BTW, a wrist staccato scale like that is exhausting, so you cannot work with this more than a few minutes. This detail level might seem terribly time-consuming and energy consuming, but the point is that you work for minutes, not hours, with the issues. And yes, on this level it is not difficult to stay focused.
Hope this was of any kind of inspiration and help.
Topic: Focus?