Learn Dutch: Ik motiveer niet alleen mezelf, maar ook de opdrachtgever. - I motivate not only myself, but also the client.

We use the emphatic form mezelf because there is a direct contrast in the sentence. You are pointing out that you motivate not just yourself, but also someone else (the client). Whenever you want to put heavy emphasis on the reflexive pronoun or contrast it with someone else, you add -zelf.

How do you use the niet alleen ..., maar ook ... structure?

This translates directly to 'not only ..., but also ...'. Just like in English, you use it to connect two things and emphasize that both are true. In this sentence, it connects the two people being motivated: mezelf and de opdrachtgever.

What are the parts that make up the word opdrachtgever?

It is a compound noun made of de opdracht (the assignment, task, or commission) and de gever (the giver). Literally the 'assignment giver', this is the standard Dutch business term for a client, principal, or whoever commissioned the work.

Does the word maar affect the word order here?

No. The word maar (but) is a coordinating conjunction, meaning it doesn't push the verb to the end of the sentence like some other connecting words do. Furthermore, here it is simply connecting two nouns (mezelf and de opdrachtgever) rather than two full clauses, so there are no extra verbs to move around anyway.