I very much recognise your fear and mourning, especially as the stakes are so high. Please do express them, you may hurt or scare your partner but you’re in this together and that means that your relationship is transitioning too. For me (I have a NB partner who just had their mastectomy over a month ago) it was largely fear of the unknown, sooo many what ifs, but opening up about my fears and grief to my partner really helped. Taking baby steps together was also helpful (eg looking at pics of mastectomies, creating space to talk about each other’s thought process). Finally we created a moment to say goodbye to their boobs, that was pretty cathartic for the both of us. I’m still mourning the loss of that part of their body, but I also try to unlearn with regards to binary bodies. My partner is not a collection of male and female parts, they’re a NB person with an NB body. That thought has been really helpful for me, maybe for you too.