You can take the route I took my junior year. I was a walk-on soccer player for a D-1 team. I was expected to start and clearly had a better camp than a freshmen they recruited that also played my position.
Didn't matter, coach liked him more for whatever weird reason and made a promise to his parents that he was going to start. This freshman told me this to my face.
Depth chart was released and they put him above me. This coach rarely ever played his second-stringers so I was going to sit the bench all year as a junior behind a freshmen. F-that.
I didn't even bother showing up for practice that afternoon and didn't even tell the coach I was done. I just left. The coach was a major douche anyways who was fired midseason. If my coach was a decent person and if I legitimately lost my starting role, I would have stayed on the team.
As for the freshman who took my spot, he didn't even last a year on the team. The new coach saw that he sucked and kept him on the bench.
Sorry so long. If your coach is a good and ethical, just go to his office and explain to him why you need to leave. It happens all the time. Good luck.