This is going to be a long one.
My girlfriend and I have been dating long distance for around 2 years now, and early on in the relationship we had talked about our pasts etc. as couples often do. During that conversation, I mentioned an ex of about a month that I dated at least a year before dating her (and there was another girl of 3 months in between the relationships) and apparently spoke too highly of her - I definitely have a tendency of putting my foot in my mouth but I stress I DO NOT think highly of the ex at all.
Fast forward two years and my girlfriend is now letting it get to her to the point where it's ruining everything. We don't get to enjoy our time any more because whenever we do something, she flips out - genuinely can't remember the last time we had a good day. She has had several panic attacks on Skype/during calls, talked of depression, anxiety and even more serious issues than that, demanding answers that I've already given her. She won't even believe me when I explain to her that it simply isn't true that this ex meant anything other than a University fling, nor did I understand why she's letting something that occurred before we had even met affect her in this way. I ask her why she's neglects all of the significant things I say about her every time we speak? She can't even justify why she's with me, and this is beginning to chip away at me severely.
Now it may seem obvious that this is not possible to easily fix, and given the impact it's been having on my University life (late nights arguing in circles), I've thought about leaving. But the issue is that I very much love this girl, and it kills me to see her struggling this badly. Furthermore, she tells me that without me, she's alone (her family and friends are never around, and it really peeves me off) and therefore can't leave out of crippling fear of descending into chaos - she's really stuck and feeling inferior is making her depression worse and worse, which terrifies me. This confuses me massively but increasingly points in the direction of depression. Any advice on how to juggle this? I'm extremely worried about out relationship, but even more worried about her. Sorry it's long, any response would be a huge help.