I've made a huge mistake during my playtrough. I've become a vampire on a character that I'm planning on keeping at 100 mana points. I've been cured but I cannot seem to cancel the questline. I wanna be a vampire slayer. Is there any way I can go back without reloading another save? I'll lose ALOT of levels if I reload a save. I'm usually strongly agaisnt cheating, but if there is any console command to become a slayer once again, please tell me.

cancel what questline?

The Dawnguard questline as a vampire. I think I may have found the solution though, I unticked Dawnguard from the data files on startup and I've reset my progress on the DLC. Mods, please lock/delete this .

well if your cured you should be able to go back to Fort Dawnguard?

Unless you were well in the middle of doing the quests for the vampires.