What does Juliet's soliloquy at the beginning of Act 3 reveal about her feelings and state of mind? Why?
The purpose of a soliloquy is to reveal to the audience what is going on in the mind of the speaker.
Much of Juliet's soliloquy echoes many of Romeo's comments in the balcony scene (Act II, scene ii). Shakespeare revisits the theme here of love being associated with light and heavenly bodies. However, we also see just the opposite as well. In the balcony scene, Romeo wanted Juliet, his "sun", to rise; here, Juliet is asking the sun to quickly set and bring on night so that she and Romeo can enjoy their wedding night.
In fact, Juliet suggests that Romeo is so "bright" that, if made into stars, people would begin to worship him rather than the sun.
The optimism and imagery of this scene is a clear allusion to the balcony scene and serves as a springboard to the disappointment and heartache associated with the news of Tybalt's death and Romeo's banishment brought in by the Nurse.
I assume that you are talking about Juliet's soliloquy at the start of Act III, Scene 2. To me, that speech shows that she is really in love with Romeo and cannot wait to sleep with him now that they are married.
The first half of the speech is about her wanting him. She says that she wants night to hurry up and get there so he can come and they can sleep together.
Then in the second part she talks about how great Romeo is and how if he were dead and made into stars people would ignore the sun.
So she's all sappy in love with him and she can't wait to star their married life together.