In addition to the fact that her beloved killed her father, I think she understands from Claudius' reaction to the play that she has participated in spying for a murderous king.

If in fact the contempt Hamlet shows toward her in the nunnery scene is justified, perhaps she cannot bear it.

My own theory is that Ophelia made the fatal mistake of thinking she could discern the truth, that she could know whom to trust (that is, her father). She is a victim of all the the deceit and spying going on in Elsinor.

Consider her snatches of song and dialogue in 4.5:

Ophelia: . . .How should I your true love know
From another one? . . .
Gertrude: Alas, sweet lady, what imports this song?
Ophelia: Say you? nay, pray you, mark.
. . .They say the owl was a baker’s daughter. Lord, we
know what we are, but know not what we may be.
. . . Pray you, let’s have no words of this; but when they
ask you what it means, say you this: (23–46)

She speaks much of her father; says she hears
There’s tricks i’ the world;
. . . speaks things in doubt,
That carry but half sense: her speech is nothing,
Yet the unshaped use of it doth move
The hearers to collection; they aim at it,
And botch the words up fit to their own thoughts;
Which, as her winks, and nods, and gestures yield them,
Indeed would make one think there might be thought,
Though nothing sure, yet much unhappily. (4–13)