"all my tears have been used up on another love"
John tried to move on.
With Sherlock gone it would be all too easy for John to go back to how he was before meeting Sherlock, after becoming an invalid. But John couldn't. He couldn't let himself become that man again. If he went back to being that man then it would be as if Sherlock never existed. It would make everything Sherlock had done for him become undone, make it seem as if none of it had ever happened. And that hurt more than the pain of moving on with his life without Sherlock.
The first year was hard. John knew he had every right to grieve, but he refused to wallow in his sadness. He moved out of the flat, because he couldn't see it as anything other than their flat. It was too hard. He didn't see Greg and Mrs. Hudson nearly enough, because it only brought up memories of the man that had introduced them. He become the man that had no friends once again. The first year was hard.
Then he met Mary.
Mary helped him move on. Helped him start a new life. Mary loved him, and he tried to love Mary with the same intensity but... he couldn't. What he had felt for Sherlock... anything else dulled in comparison, a temporary flicker like the flame of a candle right before its wick was drowned in wax.
Mary understood that he was damaged. That he couldn't give her what she gave him, so she gave for the both of them. And it was enough to keep him going. Enough for Sherlock to become a dull ache in this heart, always present but made better by the love Mary gave him. And he kept going. Kept going until he didn't have to keep going anymore, because Sherlock came back. He stopped being dead.