Does optimism conflict with preparing oneself for the worst-case scenario?

The phrase mentioned regarding optimism is not a prophetic hadith. However, optimism for good is legitimate, based on the Prophet's (peace be upon him) saying: "The Prophet (peace be upon him) liked good omens and disliked evil omens." The reason for this is that pessimism is a bad opinion of Allah, while optimism is a good opinion of Him.

It is desirable to be optimistic about the facilitation of matters and to supplicate for their facilitation. One should not strive for difficult matters, as the Sharia has forbidden wishing for tribulation and commanded seeking refuge from severe affliction (جهد البلاء), evil decree (سوء القضاء), wretchedness (درك الشقاء), and the gloating of enemies (شماتة الأعداء). The Prophet (peace be upon him) said: "O people, do not wish to encounter the enemy, and ask Allah for well-being. But if you do encounter them, then be patient." And he (peace be upon him) said: "Seek refuge with Allah from severe affliction, wretchedness, evil decree, and the gloating of enemies."

It was narrated from Abu Hurairah that the Prophet (peace be upon him) used to seek refuge from these things. "Evil decree" (سوء القضاء) encompasses matters of religion, the world, the body, wealth, family, and the end of life. "Wretchedness" (درك الشقاء) means that a person is overtaken by wretchedness in matters of this world and the hereafter. "The gloating of enemies" (شماتة الأعداء) is the enemy's joy at a calamity that befalls their foe. "Severe affliction" (جهد البلاء) refers to a difficult situation, or lack of wealth coupled with many dependents.