Igotta answer these questions about the feelings i felt after reading harry potter and the halfblood prince and i don't remember half the book and don't have it with me so yea.
1. what were your feelings after reading the opening chapter(s) of the book? after reading half of the book? after finishing the book?

I can't answer on your feelings, but I can try to help remind you of the book.
The opening chapters showed Narcissa Malfoy and Serverus Snape making an Unbreakable Vow to do whatever possible to protect Draco Malfoy, who had an impossible task to complete by the end of the year. Bellatrix forced Snape to make the vow because she didn't trust him.
By half the book, Harry and Dumbledore have looked through enough of Voldemort's past to understand that he has been making Horcruxes, or splitting his soul to remain alive. He has seven of them. Slughorn told him how to make them, but didn't want to.
By the end of the book (BIGGEST HARRY POTTER SPOILER EVER HERE), Draco is supposed to have killed Dumbledore, but he is weak and afraid. Snape kills Dumbledore. Supposedly his friend, but he needed to protect Draco. This left many readers angry. The Death Eaters take over Hogwarts and Harry Ron and Hermione decide to go find the Horcruxes themselves.

My feelings in the beginning were surprised after some facts i found out
in the middle i was excited to know the ending
in the ending i felt sad after Dumbledore's death

That's something your gonna have to yourself with buddy!