This letters starts with addressing the desire for fame and ends with advice for managing the desire of the belly. Both are desires often found in the human condition.
The good news is this: you can fix the problems; it is within your control. It would seem that Lucilius, to whom Seneca is writing, is still having trouble solving these problems. He knows what to do, but fails to act accordingly. And one might even say that the real problem is that Lucilius doesn't really know the right course of action - that's he's not been properly convinced and educated in the matter, otherwise his actions would follow the course of his knowledge. Perhaps this is what Seneca means when he says "you do not know what you want."
Seneca chides him:
Your greatest difficulty is with yourself; for you are your own stumbling-block. You do not know what you want. You are better at approving the right course than at following it out. You see where the true happiness lies, but you have not the courage to attain it.
Fame: public recognition; having a well-known reputation. Fame is quite the challenge to attain it and keep it. You could try a search engine query with "the most famous person in ..." and pick a year older than your mom's birth year and odds are, you probably never heard of them. And out of the billions of people alive on the earth today, you have only heard the names of a minute fraction of them.
And if you do chase it and attain some fame and then lose it; you are actually getting a promotion in life. The same could be said of being sent into exile.
You think that this condition, which you are to abandon, is one of importance, and after resolving upon that ideal state of calm into which you hope to pass, you are held back by the lustre of your present life, from which it is your intention to depart, just as if you were about to fall into a state of filth and darkness. This is a mistake, Lucilius; to go from your present life into the other is a promotion.
Fame is worthless. You are a speck of a drop in an endless sea of atoms. Fame is nothing and meaningless, therefore, don't waste time chasing it. Remember Seneca's words:
The deep flood of time will roll over us; some few great men will raise their heads above it, and, though destined at the last to depart into the same realms of silence, will battle against oblivion and maintain their ground for long.
For sure, fame brings influence. And if you are seeking to influence others, perhaps a better way to go about it is the "pay it forward" method - or the "word of mouth" method. By being a good person and helping others, you become a part of another person's memory. That person, in turn, may turn the same good deed by helping another person and the cycle may repeat itself. If your desire for influence stems from this, then perhaps that is a good thing to pursue.
But in the case of innate ability, the respect in which it is held increases, and not only does honour accrue to the man himself, but whatever has attached itself to his memory is passed on from one to another.
Seneca uses the phrase "innate ability." To me, this means the part of you that truly belongs to you: your attitude; your will to act and perform good deeds.
And taking a broader view of the desire for fame, but also other desires, Seneca quotes Epicurus about becoming rich (in whatever). In brief, the quote means: If you wish to be rich, subtract desires.
When you budget your money and you wish to be rich, there is more than one way to accumulate wealth. You can continue to add revenue streams or you may reduce your costs. The same concept applies to all markets in human desires. The less you have, the richer you are. To put a finer point to it, the less worries you have, the more fulfilled you are. And to not have anxiety or worry is something in your control.
Now, to address those desires that are tied to our survival; and more precisely: eating. Seneca calls these the "desires which refuse alleviation." How do you deal with these?
The belly will not listen to advice; it makes demands, it importunes. And yet it is not a troublesome creditor; you can send it away at small cost, provided only that you give it what you owe, not merely all you are able to give.
Admittedly, this is a struggle I've dealt with for a long time. I think genetics plays a big part of the source and management of this desire. Some, like a relative of mine, can eat 13 beef ribs and not be affected at all by it. For me, what I've found that works in management of this desire, is to avoid sugars and processed food. I try to eat a lot of protein and do a lot of intermittent fasting. This seems to give the belly its due without spending my life at the dinner table.