Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tristĭtūdo, ĭnis, f. [tristis], sadness, sorrow, grief, melancholy, dejection (post-class.): omnem de tuo pectore tristitudinem mitte, App. M. 3, p. 134; so id. Mag. p. 295, 19; Sid. Ep. 8, 11 fin.