The rest of this passage is very long so I encourage you to read the full letter — I only choose small excerpts that tend to resonate with me at the moment, so you may find something different for yourself.
This passage reminds me of the bit Henry Rollins does on cynicism. Although Rollins focuses more on the general notion of fear which leads to cynicism but can ultimately be combated by understanding others and building tolerance. Seneca focuses more on the fear of death portion. Where it dwells within us, and how can be combat it, that fear remains a latent force within us that is unavoidable — so why try to avoid it? To combat this overwhelming sense of fear, Seneca recommends a regular quarter-life existential crisis.
What Seneca essentially states is to make peace with poverty. Now, forgive me for my lack of philosophical knowledge but I believe the context in which Seneca means poverty isn't the developing world or urban streets that we immediatly think of. Rather, poverty is simply the lack of something. This is a huge source of anxiety within men. We look at what others have built up and we look upon it with envy. Seneca certainly isn't promoting some type of monkish lifestyle but knowing the historical contradiction of Seneca's words and action it is sometimes hard to reconcile.
So what should we do? Especially in a day and age where visibility of the individual is so prominently highlighted and easy to showcase? Hell, I don't know.
You do you. And try and find some moments of joy out of it.
What has helped me is the teaching of Carl Jung and the shadow-self . I've battled with my inner self and what I considered my morally ambiguous thoughts. Illusions of grandeur, money, women, etc. It's okay to have those thoughts and even okay to act on it. But At the end of the day, I choose to have a "moral and civic true north". So most of the time I won't compromise my principals or morals — but that's just my choice.