Aquinas addresses the virtue of perseverance and the vices opposed to perseverance in Q.137[0] and Q.138[1] of the Summa, respectively. A virtue here is "a habit that directs us to do something well, or to omit something". Perseverance allows us to avoid forsaking "a good on account of long endurance of difficulties and toils".
As a virtue, it holds the mean between the errors that flank it on either side, avoiding effeminacy and delicacy on the one hand, and pertinacity on the other.
Effeminacy "withdraws from good on account of sorrow caused by lack of pleasure, yielding as it were to a weak motion", while delicacy "is a kind of effeminacy", but while effeminacy "regards lack of pleasures [...] delicacy regards the cause that hinders pleasure, for instance toil". In other words, effeminacy shrinks from things, because of the lack of pleasure, while delicacy shrinks on account of the discomfort caused.
Pertinacity holds on "impudently, as being utterly tenacious". It resists course correction.