This passage is fairly clear and straight-forward.
rephrased: you are what you think.
Marcus goes on to advise himself of thoughts he should repeat often. You can live a good life anywhere if you live a life of virtue (courage, temperance, courage, justice). It does not matter if you live in poverty or in a palace.
His second thought he repeats often to himself is, each person is made for the interest (to help) one another. We (humans) are meant to be a community and to help each other.
(see also Citadel p. 29, 43, 103, 160, 267, 291)