My girlfriend and I both have major depressive disorder. These days I feel like I mostly manage keep my self in a good way and usually feel mentally healthy. My girlfriend however struggles more with how her brain treats her.   
Her line of work (science) is very fickle, frustrating, and delicate in nature and when something goes wrong, she feels like it ruins her entire life. I always want to be there for her and be supportive, and it's easy to work with the situations because she's almost always blowing it out of proportion. Usually it's along the lines of:  
***"I spilled a vial and now I'm going to kill myself. No one else has every lost any lab resources before, I should just give up before I do any more harm. I can't do anything right."***  
and I will usually respond with something along the lines of  
***"It's okay, you're not the first and you wont be the last. What you bring to the table at the lab is worth so much more than a vial of x solution. The lab expects to expend resources because they know mistakes happen. Tomorrow will be better and you're learning from what happened today. I guarantee you nobody is even a tenth as upset about the situation as you are with yourself right now"*** etc.  
which I know is what she needs to hear from me because that's usually what calms her down. However, a lot of the time she just continues spiraling and beating herself up. She'll start arguing with me when I'm being supportive and trying to help her manage the intrusive thoughts. She champions the voices of her irrational insecurities to the point that it seems like it's recreational to her; like she doesn't want to feel better.  
It's getting exhausting and I don't know what to do. I love this girl more than sleep and air but it makes me miserable seeing her like this all the time &amp; even worse when it feels like she isn't trying to feel better even when I'm doing everything I can to try and help her.