Try drawing with a brush and paint with one colour or a limited palette instead of using a pencil. You will achieve a quality of line that will vary in thickness which should look more interesting. You will also become more confident about handling a brush which should help you with your studio paintings.
I recommend a small travelling brush for sketchbook work, or a size 20 Stratford & York for studio paintings. The study above was painted in about 30 minutes with a travelling brush using a limited palette. As with all my watercolour sketch book work, no pencil was used.