Oh snoopy, what a horrible shock, I'm so very sorry to hear your news. But thank all the powers that be that you listened to your body and got it checked out ASAP, hopefully your early reaction will make all the difference. My sister-in-law had breast cancer and beat it (and wrote a book about it, oddly enough), and ended up making major life changes in diet and household chemicals as a result. And one of the women I work with also beat breast cancer a year ago. I know none of this is any real reassurance to you right now, but I hope you realise we are all here for you no matter what you're going through. Please don't think you have to leave the thread just because you may not for now be thinking of another child, please do stay with us if you feel up to it and share your worries/hopes/fears if you want to - we're here for you, no matter what. duggs said it very well I think.
I'm sure you will now be wondering if any of the treatments you've been on have contributed to it, but did you know that Letrozole is used as a drug to fight breast cancer, albeit mostly in post-menopausal women? And as duggs asks, have you told your oncologist/consultant about the treatments you've been on? I'm sure your family and friends will be a great support, but do think about contacting someone like Macmillan (www.macmillan.org.uk/) to help you and your family get your heads around this.
Huge hugs to you and your family, thinking of you and sending you love and support.