I most definitely have experience with this; I've been battling advanced stage breast cancer for 5 years now.
1. Try not to worry. Most lumps are not malignant. If your doctor is a good doctor and if he/she thought there was any possibility, you would have been scheduled for an ultrasound in addition to the mammogram.
2. If it is malignant, most women are diagnosed early stage. Treatment could be as easy as a day surgery to remove the tumor.
3. Even if it were to be advanced stage, breast cancer is very treatable. My oncologist has told me I could live another 20 years at Stage 4. And in 20 years, the medical advancement could be so great that it could be a complete non-issue.
4. Stay off the internet. Don't Google anything. And don't let people tell you their own horror stories. Everyone's experience is different.
5. It has always helped me when I get down about this to tell myself that worrying about it isn't going to change anything. If I have xxx amount of time to live, I want that time to be high quality, not full of stress/panic over things I am unable to control. The last 5 years of my life have been the best I've ever had.
I will keep my fingers crossed for you.