Have you only just found out that she's in a long term relationship? That's a hard bit of reality to digest. Of course our longings are not just about sex. I think ET is very complex and multi-layered.

Without wishing to be flippant because I can completely empathise with your pain; you can pretty much guarantee that a 40yr relationship will have had many ups and downs and they probably hate each other and fight on a regular basis! Seriously long term relationships can be very dull. But I do understand what it's like to imagine being the partner and knowing that you can never be that person. ET is painful.

Sending solidarity