tenacious
adj.refusing to give up or let go of something. You use this to describe someone who keeps trying very hard even when things are difficult.
adj.persistent in maintaining an adherence to a goal, belief, or physical grip. Often used to describe a person's character or the strength of a physical hold.
“She is a tenacious student who never stops asking questions.”
“The lawyer's tenacious questioning eventually forced the witness to admit the truth about the missing documents.”
“Despite facing repeated rejections from every major publisher, the author remained tenacious, eventually securing a deal that launched her career into the bestseller lists.”
Often used predicatively after 'be' or 'remain', or attributively before a noun like 'grip', 'defense', or 'advocate'.
“What looks as if it will be a cat-and-mouse story, in which a tenacious FBI agent hunts a twisted serial killer, quickly rips up any expectations.”
From Latin tenāx (“holding fast, clinging”), from tenēre (“to hold”), + -ious.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.