Struggling with my best friends depression

Melanie
My best friend has struggled with depression for a long time I do everything I can to be understanding and make her feel better but if I'm honest I feel it taking a toll on me. 
She lashes out at me for every little thing I do wrong, picks at my insecurities and has even moved on to making digs about my other friends and boyfriend. She says she does this bc her depression stems from mistrust and disappointment of other people but in our 15+ yrs of friendship I've NEVER given her a reason not to trust me. 
I WILL ALWAYS BE THERE FOR HER but I can't seem to make her feel any better and I can't seem to feel better myself. 
Any advice? 😔 ❤️
(Ps no rude comments please, I know that depression has no logic or reason but her picking on me and the people around me is really testing my patience)
(PPs she can't afford therapy and her parents don't understand depression so that's not an option)