I found the biggest (and most worring) part for me was actually getting diagnosed!
I am 35 and my GP initially told me that my lump must be from my seat-belt (seriouslly!!) because my mammogram came back clear 7 months earlier (even though I was having problems then). Not being happy with that, I had to go to another GP I didn't know who thankfully thought it was something and sent me off for more extensive testing. Guess what...it was breast cancer and I continue to have treatment.
GP's need to understand that cancer CAN happen in young women and that sometimes mammograms/ultrasounds are not the best way to diagnose young women due to the densitiy of the breast tissue. MRI's are more effective. My younger sister went to her GP after my diagnosis and he said to her "I learnt my lesson because a few years ago I had a young patient and the mammogram didn't detect the cancer. Unfortunately it was not a good outcome. Now I recommend MRI's for all of my young patients". That is a hard lesson to learn for all involved!