My mum won't accept treatment

I feel so angry, and guilty for being selfish, but I don't know what to do.

My mum is 47 and she was diagnosed with breast cancer last month (we don't know the stage, but it's grade 2). She had a mastecomy a few weeks ago and during the operation they removed some lymph nodes but they were clear. Her doctor wants her to have radiotherapy and the oncologist suggested chemotherapy too, because she's still young and it'll give her a better prognosis for long-term survival, but now she's saying she doesn't want either treatment. I feel so angry, and I feel guilty for feeling like this when really I know she needs my support, but I think she's being selfish and right now I don't even want to be around her. I know that sounds horrible. She said from the start that she wouldn't take chemotherapy, but she only decided yesterday that she doesn't want radiotherapy either, because her oncologist mentioned the possible side effects (tightening of the chest and a slight loss of bone density in the ribs around the area being treated - more superficial than a mastecomy, which she agreed to, so I really don't understand). Whenever I confront her about it I end up making her cry, and it makes it difficult for me because she's asking me to respect a decision I don't understand and that she won't explain to me.

She also has anorexia, and I think part of the reason she's refusing treatment is because she knows she'll need to gain weight to be able to handle something as physically draining as chemo. I know it's a manifestation of depression, but no one talks about it (her doctors have mentioned that she's 'slim' and might need to gain some weight, but they always say it jokingly, like they're tiptoeing around an awkward conversation) and I feel like she's prioritising vanity, or whatever it is, over her life. I've been confronting her about it for years, and I accept that it's something I might not understand, but I still think she's being selfish. I don't know if that just makes me cruel, or if it's valid, but when I try to talk to her about it I just make her cry, so I stopped trying.

I'm 20 and I just left university, and we both live in rented accommodation. I know I'm old enough to look after myself, but I feel lost. I understand that ultimately it's her decision, and her body, and her life, but I don't know if she even understands the risk she's taking. Most of the time when I ask her why she doesn't want treatment she shrugs and says she can do what she wants, or she'll say 'they've taken out the tumour, so I might not even have any cancer left'. I know there's always a risk of recurrence, but can that be true? I feel like if I don't push her to accept treatment and just respect her decision, that I'll be advocating her killing herself. She's still young, and there's no sign that it's spread (so even if there are some cells left they can be treated?), and she's giving up her chance for superficial reasons, or if I'm wrong about that, then for reasons I completely don't understand. I don't want to be angry at her, and I'm angry at myself for handling this the way I have. I know this is probably harder on her than she's letting on.

We don't have other family in the UK, and she does have a lot of friends who have been incredibly helpful and supportive, but she's moving away in a few weeks because she wants some space. She's a very private person so it wouldn't surprise me if she hasn't been totally honest with me.

Thanks for anyone taking the time to read this. Any advice or suggestions would be greatly appreciated, be as honest as you like.