Let me say thanks for a good thread.
I sense that you're asking for advice regarding how to handle the possible breakup? Just keep in mind that even if you're probably right that things may be heading for an end, it might not. You should take care not to signal that YOU want it to end. At the same time, make sure to say you understand that she'll probably want to move on due to the age difference and that you'll be happy with a FWB relationship too. Avoid making her break up because she thinks it gets too serious. But the two of you sound like a couple today, not FWBs
For all you know, she may have had a crush on you since she was 13, and be really in love with you. You could end up like the 65 year old limpdick karlhein cuckolded in his youth
Just talk about how hard it is to find people you really like to be with. Say you're really struggling to avoid falling totally in love with her. Make her think she may not have what she has with you with anyone else. It might be true
Of course, Im assuming you want to have a really long term relationship with her if you can, with all it'd mean. For once , you'd probably have to put up with becoming a dad at age 45