"The first day of winter has passed. As the rainy days are getting colder, my health condition has gotten worse.
I planned to continue working a lot more this year, but it seems like it is impossible.
I am thinking to move and get a new job, then live a lifestyle with more care for my health.
Therefore, with this as my last message, I will now retire. [could simply mean "you will not hear from me again as I will retire," and not necessarily a death letter.]
It is not a good way to say farewell, but I hope you understand.
It was short, but I am really grateful for having been acquainted with you. [could also mean "for having been your girl/boyfriend."]
Everything flows from and to God. All the best for the population." [this last line is pretty much a guess... not sure if it's a proverb or whatever, but I definitely don't get the context.]