Watch the Big Rock: Rhythm online guitar lesson by Jeff McErlain from Blues-Rock Evolution
If you can't focus, try setting a stop watch or egg timer for three minutes. Work on whatever exercise you want to master for that three minutes. It's amazing where your mind can go in that amount of time.
If you find your mind wandering, bring it back to focus, telling yourself, "when three minutes are up I can take a break." This technique got me through learning my arpeggios, in all 12 keys, through the cycle of fourths. Well, that and coffee.