Figure 1:
Three personifications of fortuna
(immediate contextual factors underscored)

1. Phoebus rashly promises son Phaethon that he may have any boon. The lad foolishly chooses to drive the sun-chariot. A god cannot break his own promises, so Phoebus attempts to give his son good advice. This advice ignored, Phoebus gives up in despair before allowing the lad to drive the sun-chariot across the sky, with disasterous results (2.140-1):

... fortunae cetera mando I commit everything else to fortune, quae iuvet et melius quam tu tibi consulat opto. may [she] help you and guide you better than you do yourself.

+ broad context of the folkloric hero's helper

2.Perseus, at his wedding to Andromeda, fighting Phineus & al.; fortuna here helps him as would any companion in battle (5.140-41)

dumque manum fortuna iuvat, Clytiumque Claninque, and while fortune helped him, Clytius and Clanus, matre satos una, diverso vulnere fudit; sons of one mother, he killed with different wounds;

+ the helper-in-battle

3. Niobe, boasting of her many children, compares herself favourably to the goddess Latona, mother of only two children, Phoebus and Diana. In particular she boasts that these many children make her invulnerable, just prior to their slaughter at the hands of enraged Latona's children (6.194-6).

...tutam me copia fecit. ...Abundance has made me safe. maior sum quam cui possit fortuna nocere, I am greater than it is possible for fortune to harm; multaque ut eripiat, multo mihi plura relinquet. Though [she] should take away many, [she] will leave me many more.

+ parallel with Phoebus and Diana, agents of fortuna/Latona