In "A&P," one would have to agree with Sammy that the world is indeed going to be hard for him from now on in.
That's because he's a romantic in an unromantic world. Life is particularly hard for such people as the world is harsh and doesn't often conform to how they expect the world to be.
Sammy's already had experience of this. He made a romantic gesture, hoping that, at the very least, it would be appreciated. But sadly, that was not the case. Not only was his kind gesture towards the three girls in their bathing suits not appreciated, it cost him his job.
Sammy has learned a very important lesson about the world. This is not a place where gallant gestures tend to be rewarded or lead towards positive outcomes. In a harsh world, a world unfit for romantics like Sammy, such gestures mainly lead to unhappy endings.
Despite his lesson, one gets the impression that Sammy cannot help being the kind of guy he is. It's almost certain that he'll do the exact same thing in a similar situation in the future, with the same results.
That would explain why Sammy's so pessimistic about the world and how hard it will be to him in the future. He won't change and neither will the world, and so, in any future collision between the two, it'll be Sammy who'll come off worst.