Искала в интернете на русском, но во всех найденных текстах информация не передает самого главного.
Дневник появился для общения с любимым человеком, тогда, когда мне с ним очень нужна была невероятная удача.

Ну и кроме того, название немного (для меня) было созвучно птице-фениксу, которая, умирая, не умирает до конца, но возрождается из пепла.

Итак, Феликс Фелицис:

Felix Felicis is a potion that is "Liquid Luck". Felix Felicis makes the drinker lucky for a period of time, depending on how much is taken, during which time everything they attempt will be successful. It must be used sparingly, however, because if taken in excess it causes giddiness, recklessness and dangerous overconfidence. It is highly toxic in large quantities, and is also a banned substance in all organised competitions such as Quidditch, along with all other methods of cheating. It is very difficult to make, disastrous to get wrong and must stew for six months before it is ready to be consumed.

Felix Felicis possibly works by providing the drinker with the best possible scenario. This usually registers in the drinker's mind as an unusual urge to do a certain action, or as a voice telling him to do so. The effectiveness of the potion thus depends on the cooperation of the drinker with the voice, for the drinker may choose not to obey whatever the voice tells him to do. This is easily remedied, however, as the potion does not single out only one scenario, but changes paths as the situation unfolds.

Felix is the Latin word meaning happy or lucky. 'Felicis' is from the same root, but declined in the genitive case. It is translated as 'of luck.' Therefore, Felix Felicis means 'Luck of Luck' or 'Luck's Luck.' or 'Liquid Luck.'

При неумеренном употреблении вызывает головокружение, безрассудство и опасный избыток уверенности в себе.
Видимо, передоз. Прекращаем.