and ending to this sentence in parallel form. "After writing all night, Sandra finally had a good introduction, but -"

''But writing the conclusion was hard.''

In this ending to the sentence that I have in your question, I have put the same pattern of the words as they are in the question and with that I have showed the idea of Sandra's writing skills.

Parallel structures are using the same pattern in the sentence with the words, clauses or phrases.

With this kind of structures people can use coordinating conjunctions and in this case coordinating conjunction is ''but''.