For almost as long as I've known my best friend, she's swung between two extremes: one week talking as though she's the most amazing person on the planet, and the next, as though she's the worst. At the moment, she's going through a bit of a rough time with long-standing family issues and pressure from university, and her down-moods are much more predominant. She's a very expressive person who couldn't keep anything bottled up if she tried, so I spend as much time as I can listening to her when she wants to talk about stuff and cheering her up when I sense it's possible. Anyway, every now and then she'll tell me that she hates herself, and sometimes she'll drop it into a conversation that had previously been relatively jokey. I can deal with most things, but this sentence always leaves me gawping at her like a goldfish. I don't know what to say.
Part of the reason for my confusion is that I've been there. Hell, I
am there. And I know that if I tell her that she's great and that she has no reason to hate herself, at best she'll think I'm just being nice and won't believe me, and at worst she'll feel completely invalidated. I want to make her feel better about herself but I don't know how, and I worry that if I do say anything in response it'll just come out as a useless cliche that will only make her feel worse.
Tl;dr: What is the best thing to say to someone who tells you that they hate themselves, without it sounding hackneyed or making things worse?