Question: Is it such that when chanting a mantra like the Great Compassion Mantra, it must be chanted accurately?
Answer: Although sincerity matters more than accuracy, especially for longer mantras with many words, they should be chanted as accurately as possible, as technically, a wrongly chanted mantra with many mispronounced words is no longer the actual mantra, just as your name severely mispronounced is no longer your name. Accurate chanting also helps the mantra to be passed down properly to future generations.
You should listen more to recordings or mass-chanting of it, while chanting along to be more familiar. That said, if you still find the Great Compassion Mantra difficult to chant, the name of Amitabha Buddha (i.e. Amituofo), though short, is already an unsurpassable mantra, that encompasses all teachings of the Buddhas too.
The Name Of Amituofo Is The Supreme Mantra
Does It Matter How Amituofo’s Name Is Pronounced?