Assuming the hot supply is really hot, all new shower controls have a high limit mechancal adjustment. On some, it is a screw, on others it is a lever. You'll need the instructions to figure it out. Normally, you have to take the trim off to reach it. Because summer incoming water is normally warmer, if you adjust it in the summer, it will be cold in the winter, and vice-versa. That's one reason why I prefer a themostatically controlled valve. Set the desired temp, and it works the same winter/summer, end of tank, beginning of tank.