Are you surprised, as if it were a novelty, that after such long travel and so many changes of scene you have not been able to shake off the gloom and heaviness of your mind? You need a change of soul rather than a change of climate.
"You need a change of soul rather than a change of climate!" That is a great line!
And more of the same from Seneca:
Why do you wonder that globe-trotting does not help you, seeing that you always take yourself with you?
because you flee along with yourself. You must lay aside the burdens of the mind; until you do this, no place will satisfy you.
He compares a the unwise soul of a person to an unbalanced cargo ship. As long as the inside stuff is not secure, the person will be tossed to-and-fro.
Rather, we ought to do the hard inner work. We must correct our inner dialogue and seek a life of wisdom and virtue. This will balance the soul. You fix the person, you fix the "bad vacation." You will no longer complain where you are in life but will live with good flow and equanimity no matter where you are.
The person you are matters more than the place to which you go; for that reason we should not make the mind a bondsman to any one place. Live in this belief: "I am not born for any one corner of the universe; this whole world is my country." ... that which you seek, – to live well, – is found everywhere.
He finishes the letter quoting Epicurus on why knowledge of "sin" is the beginning of salvation. Ignore the religious parlance and focus on the aspects of introspection and self-improvement. How can you improve yourself if you don't know what needs improvement? For this reason, Seneca writes:
Therefore, as far as possible, prove yourself guilty, hunt up charges against yourself; play the part, first of accuser, then of judge, last of intercessor. At times be harsh with yourself.
See also Letter 104.