It's hard but you just have to try to be honest. Tell him/her that you are really struggling and know you need support with feelings of depression over a long period of time , that it is no longer something you can handle on your own. Maybe make a few bullet point notes so you don't forget what you want to say or you could even give the notes to the doctor. Remember that the doctor won't be judging you, they will probably have heard people with similar issues and concerns before and they will want to do their best for you. Don't underplay the significance of it to the doctor, really be honest about your concerns. The doctor will likely ask you questions anyway so you probably won't have to think of too much to say. Don't worry it will be fine.