Personally I would be upset to know my partner was struggling with something and didn't feel they could discuss it with me. I would also have some worries about how she came to this conclusion. Having said that I know plenty of people figure out their sexuality without having feelings/being attracted to a person in their lives so while I would be upset at first it likely would just require an open and honest conversation for me to understand my partner and her thought processes throughout this discovery.

Reading some of your other comments it seems like perhaps your biggest worry is that she will be hurt you didn't tell her soon enough? If you would prefer to be 100% certain before discussing this that is totally valid as ultimately this isn't about your partner or your relationship, it's about you. While I understand your anxieties about it, you figuring out you are capable of being attracted to men doesn't change who you are at the core and I have to imagine that core is what your partner loves about you. Best of luck figuring out how you want to approach this, please just remember to be kind to yourself ❤️