Please do not wait. Yes it's absolutely terrifying, however knowing is always better than not knowing. There's a good chance it's not cancer, and then you would be relieved. If it is cancer, the sooner you know the better. My mom has had 2 breast cancers 10 years apart, and she's still there living her best life! The reason why: when she found a lump she went straight to her doctor and started diagnostic exams right away. There is nothing positive that can come from waiting to have a mammogram, the longer you wait the longer you are scared.
Tell your doctor about your fears, they could prescribe medication to take the night before and day of the mammogram for anxiety (you wouldn't be able to drive, plan on getting a taxi or Uber/Lyft). Can you go with a friend? If not, like @Lemonslice said, there are many breast cancer support groups that may offer to have a volunteer accompany you.