Recovered but triggered by friend

(This is my first time writing anything here, so apologies in advance if there are rules/things that I don’t get)

I need advice on how to help my friend and maybe also how to protect myself while doing it

Backstory: Around 2 years ago (after 3 years of intense therapy) I started getting my life back, I stopped self harming (something I did for almost 10 years), my mood stabilized, I moved to another city, and now my job is going well, I’m very happy in life generally. Of course it’s not always perfect but I have learned how to cope using different techniques from therapy. So yeah, it’s very good actually.

Problem: The thing is that now my best friend has started hurting herself (or she’s been doing it a long time I don’t know) and she’s not hiding it.

We’re very close friends and I think I know pretty much everything that’s going on in her life, and I listen a lot to her rants about things she’s upset about. I don’t want to be that person, you know the one that thinks “what do you have to be upset about” because of course her feelings are valid and real to her but I can’t understand how her “small” problems she always talks about have made her go in this direction

I don’t know how to talk to her about her self harm, I see the cuts and I feel sick, I flash back to when it was my arms, and I get angry at her for not even trying to hide it (I know it probably sounds super selfish) and when I’ve asked her about it she says “it just happens sometimes” In a way I feel like I need to protect myself and my happiness and health but I also want to help my best friend, except I don’t know how.. any advise?