The curse is that the women of Karen's bloodline will find themselves in a death loop. That woman's loop only stops when they find their true love, and the curse is passed on to the next generation. The curse will END (meaning no passing on to the next generation, no more loops) when ALL of the women's deaths add up to 10,000.
So for example, lets say the curse starts with Karen's great grandmother (not saying thats true, the curse has been going on for generations):
She dies 400 times until she finds her true love . Now its passed on to Karens grandmother and the amount of deaths left is 9,600.
Karen's grandmother dies 600 times, found true love, now its passed on to Karens mother and the deaths left are 9,000.
Karen's mother dies 2,000 times, finds her true love, now its Karen's turn. Deaths left are 7,000.
If Karen finds her true love, her death loop will end and she passes it on. BUT if Karen manages to die those remaining 7,000 deaths the curse will go away and no one else will experience the loop anymore.
and yes. Donna did die for real. Everyone who Karen killed died her father just covered it up.
So I just wanted to jump in here to say that it's pretty much implied that the only true way to stop the death loop is to pass it on, and the whole "true love" thing isn't really the way it works, or more like it's that for all of them, they didn't experience true love until they had a kid.
Spoiler:
Dulan knows this and purposely killed the FL the one time she was pregnant because he just wants her to keep suffering. This however made it so that the ML started remembering his past lives as well, it just takes a minute for it to happen, each time being sooner than the last.
Since so many are asking:
Yes, Karen is actually in a death loop, and yes her mother and the ancestors before her did as well. Her bloodline was cursed. The death loop only stops when they find their true love, and it passes on to the next generation. The curse in specific is that the women of Karen's bloodline must experience a total of 10,000 deaths and then the curse will vanish.
Karen's mother's true love is not her husband, but Karen