Wow. Wasn't expecting that level of vitriol. Obviously haven't and couldn't go into nuances of situation, but relationships are not clear cut, and people hurt. I haven't been on Mumsnet for years, and not sure I want to come back if this is the way people behave. Was not aiming to hurt or make anyone ill, just a wee bit of salve for a broken heart after many, many years of marriage. Thank you for the empathetic ones, but based on many of these responses, there are many out there who clearly have more difficulty relating to others than I do. Good luck in your relationships if this is how you treat people - any yes, I realise I am going to get shitloads more condemnation now. Oh well..
OP, I have been there and it fucking hurts. I feel your pain. That is why I am not going to call you childish or tell you to grow up.
However, I would use the time that he is away with her to mourn the loss of him, and your relationship with him. Cry, if you need to. Shout, if you need to. Get drunk, if you need to. But let him think that you are indifferent. If you live together, change the locks. Go make an appointment with a solicitor, if needed. But please know that you need to make sure it is over.
The pain of infidelity, especially when it is rubbed in your face, is soul-destroying. But please, please look after yourself and start making plans for your future.
You will not always feel this level of pain. But you will get through it. I will be thinking of you, op. It's shit and it hurts. Take care of yourself.