Need a woman's help

Hey so I'm having a little problem...... I am heavily sexually deprived and my SO doesn't want to have sex anymore...... She is just never in the mood and this goes on for months... We have talked over and over and I am just told to wait until she is ready.... But now I am just sexually frustrated..... I wanna ask her if I can have a mistress but idk if that would be crossing a line...... Any advice??