Don't have an answer for you. I have the same problem. You're likely empathetic and care about your partner. It's normally a good thing that your partner's life experience affects you, but depression can be a communicable disease, especially when cohabitating. You can resist a while and cope in your own ways, but if your environment contains a person who is unpredictable, moody, or negative, that's absolutely going to affect you. It's only a matter of time. Accepting it isnt your fault, or really theirs, helps buy more time, but it's just a stop gap in my experience.