The speaker of the poem starts out by telling us that he's just a little down, no big deal. As the poem goes on though, we start to get the feeling that there's something more than a little "sadness" (line 7) here. The phrase "endless toil" makes things sound pretty bad, and we sort of get the feeling that this guy is hating life. We're not sure if it's a specific thing that bothering him. Maybe it's just the daily grind. Whatever it is, he's looking for a way to escape.
Who, through long days of labor, And nights devoid of ease, (lines 29-30)
Here he imagines that the "humbler poet" he's searching for also had a tough life – not just in the day time either, but at night too. These "long days of labor" are a nice echo of the "endless toil and endeavor" that he mentioned earlier (line 23). The basic message is that life's tough all around, no matter who you are. Just getting through the day is hard, and sometimes the nights can be bad too. We imagine that most of us could get behind the idea that life involves a fair amount of suffering. Still, there are things that make it better. Like poems!
The restless pulse of care, (line 34)
Here's another image of suffering, of the way that life's troubles can stir us up and make us feel antsy and unwell. "Pulse" is a neat word to use here. It makes us think of the pulse in our veins, but also of the way that troubles or bad news (care) can roll in on us like waves.
And the cares that infest the day, (line 42)
Another strong word choice from Longfellow. "Infest" is just such a nasty, gnarly word, packed with gross images of rats and roaches and pests of all kind. If someone told you that you had an infestation, you'd want to get to the doctor or the exterminator quick, right? This is a great example of how a single word can drive home a theme like suffering. We get a quick, visceral image of how much it can stink to be alive, to have to slog through the day.