Studying for 15 min then having a 5 min break is not very effective of your time, especially this close to the exams.
Im not one to talk, i take a lot of breaks myself or simply distracted most of the time, but you should be studying for a longer period of time.
Try creating a plan for yourself before you begin any studying. Set out your goals of what you want to achieve that day, and estimate the time it will take complete, wether it be an exam or simply revising something. Then, stick to it. I would say that after each session, take about 10-30 min break. Nothing more. DO NOT allow yourself to exceed your time or you will fall behind on work.
And as for the motivation part, this is vce. All the motivationI had has disappeared throughout the year. But, try thinking about writing your last word on the last exam and your excitement then. That's all thats really keeping me going.