First, let me offer you a tremendous hug...and another. We have all been where you are, and for what it's worth, your feelings seem perfectly normal. The days after my diagnosis were the most frightening, overwhelming and angriest of my life. You WILL get through this, there is simply no choice. Allow yourself to feel what you are feeling, painful yes, but let it happen. Focus on what you can do, take charge of what you can take charge of. Reading, researching, writing, comparing notes....all things you can take control of. Reading her emails...you can take control of that too. Most of us can tell you a story, in fact I think one of our beloveds named it "The Golden Boob" award to the most insensitive and idiotic thing that someone else did or said to us.
My hunch is that your "friend" was trying to join you in your anger by sharing her horror story with her boobs too. No comparison, we know...but to her, and it may be "all about
her", but I bet that she was trying to show you too how boobs are not all they are cracked up to be. One suggestion is to email her back and simply let her know that while you know she is suffering in her own way, you feel that her issues are just not relevant for you at this difficult time and in fact, that her emails are adding to your distress, and that you will contact her when you are up to it.
As for a therapist, go for it. Depression is pretty common with a diagnosis of cancer, and not sure they will lock you up...but if your feelings become so overwhelming and life consuming, and last for more than a week or 2, it may be time to seek professional help. No shame, just that some people need a little help now and then.
Give it time, you will come to acceptance...try to set reasonable goals. It may not be reasonable to "be over it" tomorrow, but try to replace the crying with a constructive activity. Or if you cried for 2 hours yesterday, maybe tomorrow can be less. Set a realistic increment to try and reach, but do allow yourself to feel what you feel.
Hang in there!