Having to put my 16 year old dog down on Wed

(The pic is from a few years ago)

Harry is my childhood dog, he is now 16 years old and I believed he had at least a good year ahead of him. Yesterday, he had a fit which looked like a seizure, he threw up and was struggling to walk for awhile. He is a very old dog, I am aware of that, but I don't know how I'm supposed to cope with his death. We called up the vets and now we have decided its best to put him down on Wednesdau instead of letting him live on a tiny bit longer in pain and suffering. Tomorrow we will take him for a long day at the beach, one of his favourite places to be but like I said, on Wednesday he will be put down, I decided I want to go to the vets for it, to be there for my pal, but I generally don't know how to cope with his death. He is the first dog I adore who'll die.