Pertinacious
PERTINA'CIOUS, adjective [Latin pertinax; per and teneo, to hold.]
1. Holding or adhering to any opinion, purpose or design with obstinacy; obstinate; perversely resolute or persistent; as pertinacious in opinion; a man of pertinacious confidence.
2. Resolute; firm; constant; steady.
Diligence is a steady, constant, pertinacious study--
[This word often implies a censurable degree of firmness of constancy, like obstinacy.