I found out that accomplishing one's goal is a difficult thing.

Literal

Goal [obj-を] accomplish [quotative-という] thing [topic-は] difficult [quotative-という] thing [subj-が] was-understood.

遂げる ('accomplish, carry through') is a literary/formal verb. The sentence stacks two ということ frames — 目的を遂げるということは (the fact of accomplishing) and 難しいということがわかった (understood that it's difficult). Somewhat clunky with the repetition.