I let Serana back to her home, and after proceeding through the conversation, I chose to become a vampire lord, and once I made the choice, Harkon bit me, but rather than becoming a vampire lord, I just die, and start back from when I last saved. How do I fix this?
If you are playing in PC:
BEFORE talking to Harkon, open the console type
"setstage DLC1VQ02 40" and followed by
"setstage DLC1VQ02 180"
If you do this he will say you have forsaken the gift, he will offer you the gift again, accept it and you will not die when you are biten
If you are playing from PS3/XBOX:
Go behind a table before getting bitten. He won't reach you in time if done correctly, and the screen should fade to black after a short period of time. Following this, everything should continue normally.
I dont know if this helps, but I glitched the same way. I had vampirism disease, happened the same way, i just died. Maybe the game glitched because I was already diseased? Try to see if you're not infected, if not just reset the quest.