As soon as your permanent partner falls in love with somebody your relations are not amateur any more, but rather they get a status of professional ones.
It is because amateur level of sorting out the situation does not suggest ecological options: you can “split up”, you can “suppress your pain”, you can “force him/her stop it” – none of them will bring in pleasure, let alone development.

History does not go backwards, that is why you will not be able to return your partner into the same river.