Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
obstĭnātĭo, ōnis, f. [obstino], firmness, in a good and bad sense; resolution, steadfastness, determination, inflexibility, stubbornness, obstinacy (class.; syn.: pertinacia, contumacia): quae ego omnia obstinatione sententiae repudiavi, out of adherence to my principles, Cic. Prov. Cons. 17, 41: animi, Sen. Ep. 94, 7: fidei, Tac. H. 3, 39: taciturna, obstinate silence, Nep. Att. 22, 2: inflexibilis, Plin. Ep. 10, 97, 3.
In plur., Tert. ad Nat. 1, 17.