Sevenofnine, I could have written your post 15 years ago. I, too, have dense and cystic breasts. I felt the same way you did when I had my baseline. I still flip out every time I go for my yearly - and I'm 53 now.
It sounds as though you are going to a breast center, which is great. They see many, many dense and cystic breasts on a daily basis. They know what to look for.
I have a friend who is a radiologist at a breast center where I live. So every time I go now, she orders an ultrasound for me because I have such issues with cysts (although, after menopause, my cysts have declined dramatically). So I go, have the mammogram, wait, pray, wait, get called into the examining room, wait, pray, then she comes in and does the ultrasound. Even though it's unbelievably nerve-wracking there is nothing like the feeling of knowing that I have been thoroughly checked and am okay. I am very grateful that she takes the time to do this for me.
I also have a very close friend who is the exact same way as I am when it comes to health issues. She asked about getting a prophylactic mastectomy because her fear was just as great as mine. She found out that they don't just cut off your breasts, but also take out lymph nodes, cut through muscles etc. It's more involved than she thought.
For me, with tests like the mammogram, my anxiety and obsessiveness just builds and builds until I go ahead and get it over with. My advice is that you should just get it done. Get it behind you. Otherwise it will become an obsession. I am currently dealing with being completely focused on worrying about uterine cancer - so much so that my gyn has scheduled a painful test for me to next week so I can put my fears at rest. But this was after, literally, one complete year of worrying about it.
Best of luck to you - let us know what you decide to do.