I try to practice 30 minutes a day. While practicing I record what I play and listen to it – so I don’t inhale and exhale 15 minutes straight through.
It makes a difference if you breath shallow (chest breathing) or deep (abdominal breathing) while playing. If you breath shallow you tend to get dizzy because of the higher breathing frequency.
Try to switch to deep breathing using your diaphragm. This breathing helps to play more relaxed.