In order for you to be the object of harassment by your employer, you must be able to show that the employer's actions in making you the only employee who performs co-worker massage, is based upon a discriminatory motive: race, color, nationality, religion, sex, sexual orientation, pregnancy, age or disability.
If you cannot show a connection to one of the above-described classifications, then there is no harassment, and the employer is free to require that you perform an unreasonable share of the lower-paid massages.
For example, if you are male and all of the other massage therapists are female, then that would demonstrate a possible discriminatory motive. In which case, you could file a complaint with the Department of Fair Employment and Housing (DFEH), at this link.
I'm a bit concerned about the statement that you are full-time commission. The employer may be violating the wage and hour laws. If you would please describe a typical day's work, that could help me determine if there is something possibly unlawful taking place.
Thanks in advance and I hope this helps.