Whatever you play, practice it with a metronome at a very slow tempo, and when you are able to play it 5-10 times in a row with no mistakes, bump of the speed a bit and practice until you can do it at that speed 5-10 times with no mistakes. Keep this up until you have it at tempo.

Practice the stuff that you DON'T know. Don't waste time practicing stuff that you can already play. Work on things that are difficult for you, and continue to work on them until they become a strength.

Play with other people, and gig as much as you can. I was a music major in college, but when I first got there I didn't even play bass - I was a piano guy. I got tired of not being able to find good bass players to I started learning bass. I learned literally on the gig. There was a VA hospital that let us come in and play for the patients on a weekly basis. We did it for free, as a jazz quartet. We sucked because we were all learning to play, but the patients didn't care because they were happy to hear some music and have us come in to visit. And by the end of the first semester we were sounding good enough to get calls for paying gigs. By the end of the second semester I had switched my major to bass performance and it was my main instrument. Playing with other people (and in front of an audience) will drive you to get better. It will improve your time, and your ears. It will greatly improve your dexterity and stamina. Find some cats and start playing!