Hello there,

There so many fields in my guitar playing which I would like to improve: improvisation, music theory, songwriting, technique, etc. Also there are so many songs which I would like to learn because there is so much good music out there.

Obviously you can't do everything, so how do guys stay focused?

But really, don't do those. All this stuff takes a really long time to build on, as you probably already know. I just take it bit by bit, practice X one day and practice Y the next day. Also, just noodling (improvising) on your instrument with some sense of musical coherence also helps to build on most of these things, or so I have found in my case.

But I'm also pretty stupid, so don't listen to me too much
I think it's important to be realistic about how much time you have to practice and plan around that picking something from each area you want to practice and dividing up your time.

John Petrucci has really good ideas when it comes to practicing and I incorporate this into my routine also, I put some links to his articles below maybe it will help you too.

I agree with previous posters. You need to prioritise and not get overwhelmed at the enormity of the task at hand. Getting a good teacher is very good move as they should give you a series of specific, well defined tasks to focus on; gradually moving you nearer to where you want to take your playing.
Central to this is improving knowledge of theory ... it gives more options for improvisation and composition, and helps understand what other players /writers are up to.

And underpinning all that are intervals ... which also reduce learning and improve fretboard navigation. Learn their shapes (there are so few of these they can be mastered in around a week, and they crop up everywhere (scales, chords, melodies)).

You may find these lessons useful here: http://www.ultimate-guitar.com/lessons/the_basics/drastically_reduce_learning_time_with_intervals_part_3.html (just follow the link back to part 1),

Good luck, Jerry