I recommend "A Guide to the Good Life" by William Irvine. The title is a bit off-putting, but it gives a good survey of the philosophy and history of stoicism along with good examples of applying it to modern life. After reading it, you might have a better sense of which primary sources will be most relevant to what you're looking for.

Little to none of his work has survived intact, so what we know is mostly through other people. IIRC, a lot comes from Seneca and Marcus Aurelius's auto biography "meditations". Searching around on Seneca, his letters on the topic have been collected in a book "Letters from a Stoic"