Be honest and upfront with your doctor, as much as you are able. Try not to be nervous about saying the wrong thing or not being able to explain yourself; medical professionals are used to getting the full and true story from people who are not used to telling the full and true story. They will take you seriously. When you tell them you are thinking about suicide, they might ask you if you have a plan for it. Suicidal thoughts are always a serious thing, but there are degrees to how urgently serious they are, if you see what I mean.

Know what? You could say exactly what you wrote in your first post: "I'm nervous about this because I don't know if I'll be able to explain what I mean, and I worry about saying the wrong thing and coming across wrong". Or something like that. They will understand.

Good for you for getting the appointment. Keep us posted?