Sorry to hear about your situation. It can be really tough when you're with someone with mental health issues, so thank you for staying for so long. With depression, you can get periods of higher irritability and anger, which would explain the changes in mood.
However, it does concern me that this is taking its toll on you. Of course, each relationship has it's tough times, so this may be a case of weathering the storm and supporting her at your expense. I also think that your approach needs to be caring but firm - let her know that you're saying this because you care for her and want the best for her. However, if you feel that a relationship is taking too much out of you, then there's always the option to leave - don't feel you have to stay if misery and pain is all you're feeling in this relationshipx
Much love <3