Mom Adamant About Quitting Chemo

Hi all, new to this place.  Didn't know where else to turn for advice.

My mother was diagnosed with breast cancer earlier in the year and had surgery to remove it (stage 1, grade 3).  Everything went well and chemo began shortly after. 

For about a week after each treatment, Mom feels pretty bad (nausea, upset stomach, insomnia, etc.), which is to be expected.  Eventually, she feels better, usually just in time for her next treatment. 

She's done four rounds of chemo already, with two more to go.  This last round was a different kind of chemo though; less nausea but more pain in her joints/muscles/bones.  Today was an especially bad day for her.  Mom says she hasn't had a good night's sleep in months and I think she's getting depressed. 

Anyway, she's mentioned on some of her bad days that she wants to quit going to chemotherapy, but we (the family) manage to convince her to keep going.  This time seems different though; I think it's because of how much pain she's been in coupled with lack of restful sleep.  But either way, she's adamant about quitting chemotherapy. 

We're all really supportive of my mother, or try to be anyway.  I can't imagine what it's like to have breast cancer or chemotherapy (I am her 25 year old son), so I can't really understand first-hand the pain and discomfort that she is in.  But she only has two chemo treatments to go and it's in her best interest to do all six treatments yes?  She seems to think she'll be okay just doing the four, but that's not how it works, is it? 

I love my mom so much and I wish she would reconsider.  Does anyone have any advice for helping her stay on track?  I respect my mother's choices, it's her body, but I just feel like giving up on treatments is wrong, and might not effectively remove any lingering cancer cells.