your not letting your muscles rest your working them to much,im 17, 6ft and 15st,
do all the big muscle groups once a week n small ones twice a week,and stay with 3-4 exercises per muscle group and 8-10 reps per exersize,i would say only do cardio 1 or 2 nights a week. this is wat i do..

monday: triceps - shoulders

tuesday: back

wednesday: biceps - forarms

thursday: chest

friday: triceps - biceps - forarms

saturday: legs

and just try n eat 7-9 meals a day with over 300g of protein and 600g of carbs a day and you will soon start building muscle.