Hello,
I'm not sure if these lines belong to Mevlana, known as Rumi in the Western world.
More or less, you understand the meaning behind the lines I guess, I can only provide a similar translation maybe in a more proper English. Please also note poetry and aphorisms can gain more meaning as one contemplates on the sentences.
Sen düşünceden ibaretsin, - What you consist of is thought
geriye kalan et ve kemiksin, - Bone and flesh is the rest. (of what you consist of)
gül düşünürsün gülistan olursun, - You think of a rose and become a garden of roses
diken düşünürsün dikenlik olursun.. - You think of a thorn and become a patch of thorns
In other words, you are what you think because what humans consist of is Thought.