Hello, I'm new to this forum

I'm writing with concerns for my girlfriend. We have been dating for more than 2 years. We are very happy together. But everyone once in a while she will get depressed. She constantly suffers from some anxiety but sometimes certain events will trigger a spurt of depression. Anything from a conflict with an ex-friend, parent, ex-boyfriend or even a traffic violation. I know it comes from a few very tragic events that happened to her earlier in life. But when she gets depressed she talks about how its not worth working for anything any more, she hates this world, she feels like an outcast, and that she is possessed by this dark cloud of evil, sadness, confusion, torment, rage and she doesnt know how to get it out. She is scared and her mind moves a million miles a minute. She doesnt feel comfortable and feels pathetic. Since we've been dating she has been having fewer episodes and the episodes dont last more than a week or so. But i feel like it is always in the back of her mind waiting to come back out. She has gotten professional help before and says she needs help again, but when i offer to get some help she acts disinterested or feels like she doenst need it anymore. And then she snaps out of it and everything is fine again for several months. I'd say she gets like this every few months on average but the mild cases only last a couple days. but it hasnt been this bad in almost a year. I just dont know what to tell her anymore when she feels depressed.