What are the ways to overcome clinical depression and mood swings?

Hello doctor,

I am a 20-year-old male and have a girlfriend. We have been dating for almost three months now. She is suffering from clinical depression. She affects me a lot. A slight change in her mood, noticed by me, puts me in a very bad mood, and I cannot do anything for a long time. I care a lot about her. She loves me. But at times, I feel that she does not care for me. She is 19 years old and tells me that she loves me a lot, that I am perfect, and that she made the best decision by dating me. I accompanied her to a doctor recently, even though our exams are barely ten days from now, and I have tons to study. I could not study the entire day. She did not show a bit of gratitude. She did tell me that it was an amazing date. Later that day, she had chocolate, and I asked her to get me one from the shop right before us. She straight away told me, "no, go get your own." I asked her twice, yet I got the same response. Her response hurt me; I texted her later, again getting the same response. I wish she could understand. I do not want to irritate or do anything that affects her. But I need to know a way that does not make me so vulnerable and affected by everything that happens with her.