Yugong moves mountains
I'm surprised this didn't come up earlier, but 愚公移山 (Baidu Baiki; Wikipedia) is one of the most famous 成语 and it's about perseverance. It tells the story of Yugong who moved mountains piece by piece, and succeeds because he doesn't give up. There are a lot of variants of the story. Wikipedia describes is like this:
The myth concerns a Foolish Old Man of 90 years who lived near a pair of mountains (given in some tellings as the Taihang and the Wangwu mountains, in Yu Province). He was annoyed by the obstruction caused by the mountains and sought to dig through them with hoes and baskets. When questioned as to the seemingly impossible nature of his task, the Foolish Old Man replied that while he may not finish this task in his lifetime, through the hard work of himself, his children, and their children, and so on through the many generations, some day the mountains would be removed if he persevered. The gods in Heaven, impressed with his hard work and perseverance, ordered the mountains separated.
There much learning materials available about this 成语, such as a ChinesePod episode, various YouTube videos (e.g. Gingle ALE and 听故事学中文), Chinese at Ease, etc.
So maybe we can complete the sentence like this:
My pronunciation [is] poor, but like Yugong moving the mountains, if only [I] persevere, sooner or later [I will] attain success.