stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T S Eliot; "men tenacious of opinion"
Someone who is pertinacious continues trying to do something difficult rather than giving up quickly. continuing to believe something or to do something in a very determined way = tenacious (pertinax, from tenax; TENACIOUS)
Holding or adhering to any opinion, purpose, or design, with obstinacy; perversely persistent; obstinate; as, pertinacious plotters; a pertinacious beggar