How do i abort quest Witcher 3?

Stuck on Witcher 3 mission i made a mistake doing the Last Wish quest which is a level 15 quest. The problem is i'm level 9 and the boss is very difficult to defeat this is my first time playing a Witcher game btw so i didn't pay attention to the level requirements
for the quests until now. I'm now stuck on the mountain with no choice but to fight the boss and i can't load from a checkpoint or save so that i can level up and come back to do it later. Is their any way to abort the mission. Shouldn't these missions be
locked in the first place for people that don't have the required level to them, seems dumb that they let me do a level 15 quest in the first place.

:)

 

Companions' Purity Quest and Glenmoril Witchheads glitch.

SPOILERS AHEAD.

I have finished the Glory of the Dead quest. I am trying to start the Purity quest and then the rest of the Companion's quests. I was told I have to do miniquests and I have literally done 50+ miniquests from both Aela and Farkas. How do I start
this quest?

My other problem is Glenmoril Witchheads. At some point in time, I got rid of them. I don't know where they are now either. I slaughtered all the witches and took all of the heads and now they're all gone. What do I do? Do I need them to start Purity?
I want to get rid of being a vampire but now I can't and it's extremely annoying.

 

The Naked Nord Quest

So I just started the quest "The Naked Nord" in Deshaan and Evigna the Witch isn't appearing thus, I cannot talk to her to continue the quest. Has anyone else had this problem and know how to fix it? Thanks for the input ahead of time!