So sorry that this is happening to you .I know how you feel as I have a similar situation , even the March birthday. Married 31 years.
But due to other circumstances, he is still in the house . No OW involved at the moment . Believe me , it is hell on earth and this is 2 years down the road.
Guess what I am trying to say , is that him being out of the house is a good thing. Please do not let him back when it all goes to shit for him as it will eventually do.
You have the upper hand here, he has left the marital home. When you feel strong enough , please seek some legal advice , even delegate this to your older kids for you.
Everything you are experiencing is normal under these circumstances . The shock is so traumatic, your body reacts too. But I assure you , it will pass . Just be kind to yourself and if necessary , phone your GP for some advice and help.
It is hard to imagine life without him as it has been so long. And I know it is so overwhelming at this point to see any future by yourself. But you wont be by yourself , you have kids, grandkids and they have your best interests at heart and are there for you . They are worth more than a lying cheating partner.
The OW sounds like a prize catch ! Ha ,all he wanted was a bit of fun , Illicit Encounters , how pathetic. Now he is stuck with a crazy , nasty bitch. Definitely not Juliet!
Everyone is telling you that you will be OK ,it is true. I found it hard to believe that I would be normal sometimes, but it does happen.
I actually did smack him across the face . Not advocating it , but damn it felt good!