How do you let go and move on after a long long marriage or relationship

You allow yourself to grieve; there's no set way..only time will make it easier, and so, as someone said, one day at a time. One hour, if necessary. Be kind to yourself.

You process the hurt, you come to the understanding that it happened, you stop laying blame, and/or taking all the responsibility, and you come to the place of forgiveness.
Which is not forgetting, or letting the other off the hook. Its releasing the hold that the hurt has on you so that it is no longer in control. You forgive in your heart, give it to God, and set yourself free of that event. Only then can you move forward in freedom

I'd echo @IBHappy response.

I found i hid away for a long time.
Kept myself to myself and i found it quite isolating if i'm honest.

So. Get out there !
Go do all the things you wanted to do but couldn't because you had somebody else to think about at the time.

And don't worry if you discover certain things aren't for you.
I found i got bored of pubs and clubs etc. very quickly.
You may have to try a lot of things on before you find a 'fit'.

One day at a time. Keep yourself busy. Lots of talking with a good friend.