The thing to note here is that while the old man was victorious over the great marlin in terms of ultimately getting it back to shore, the victory was hollow in that the meat had been eaten by sharks en route back to shore, leaving him with just a skeleton for his efforts. It was therefore a victory that didn’t come with any profits for him.
One of the primary things we are told about the old man’s state of being when he returns to the shore is that he is absolutely exhausted. This is completely believable because he hasn’t slept in at least two days by this point. While attempting to head up the shore with the mast and furl the sail of his boat, he winds up falling down under the weight. Even after everything that he has been through, he finds the patience to lie still where he has fallen until he has gained his strength.
For the old man, the biggest relief is surely the return of Manolin and the boy’s promise that he will not be returning to sea alone. It is this, I believe, which gives the man the peace of mind he needs to get to sleep.