You will have days where you do not get to practise or it all goes wrong and you cannot focus.
It can be easy to get so busy in the day that there is no time or energy left at the end of the day to sit down with your instrument. That is ok! Yes, in an ideal world practise everyday is fantastic but some days it is just not possible. I spoke with one parent a few days ago who was telling me her son had 25 pieces of homework in 1 week. With that level of intrusion into time spent at home, something has to not get done.
For adult students after a long day at work, traffic and the requirements of life sometimes all you want to do is sit down and stop. That is perfectly ok. It is actually better not to practise than to spend time trying to focus and making more mistakes and problems for the next day. There are weeks that it can seem like all you do is turn up for your lesson; that is ok. It is worth remembering the story of the birds and the water jug. You can read the story, and try the interactive features, on this website http://read.gov/aesop/012.html
The fable is made the subject of a poem by the first century CE Greek Poet Bianor. The story concerns a thirsty crow that comes upon a pitcher with water at the bottom, beyond the reach of its beak. After failing to push it over, the bird drops in pebbles one by one until the water rises to the top of the pitcher, allowing it to drink. (https://en.wikipedia.org/wiki/The_Crow_and_the_Pitcher ) While I do not suggest this fable for the ingenuity usually attributed to it, I do find the notion of a skill is gathered one small piece at a time. Everytime you add a small amount of practise, or use, to this skill it improves. Even if it is only once a week at your lesson.
There is a lot of truth in the old adage "It means that if you don't continue to practice or use an ability, you might lose that ability." When I was younger I used to be good with cars, changing engines, wheels and exhausts etc. I was a confident driver and at times raced a little. However 30 years of never touching a car other than to drive it, has ensured by current mechanic skills are memories and an enjoyment of talking about how the new cars will never be as good as the old ones. If I had continued, I would have repaired the crack on my bumper by now! Even messing with my motor once a week would have helped. Now, I used to be good with cars. To return to how I started this musing. You will have days where you do no get to practise or it all goes wrong and you cannot focus. The important thing is not to stop trying, not to stop pushing and not to stop the one thing that will hold you accountable (ie your lesson).
It is OK to skip a day or more when life is too busy, but to stop learning all together is a sad day and you don't really know it's happened until it has passed and you look back and say "I used to play xyz.... I don't know what happened life just took over." ;
just as I can say "I used to be good with cars but .... I don't know what happened life just took over."