my kryptonite 

Used to refer to something that is your ultimate weakness, that which renders you powerless...in reference to Superman...whose only weakness is kryptonite.

Overthinking is my kryptonite. Or Chocolate is my kryptonite. Make sense?

my kryptonite by happythoughts October 14, 2014

are my kryptonite

It's a way of saying that something specific is your weakness. I.E. Stating that you can't help but love it, or that it's your biggest addiction.

Puppy videos are my kryptonite, I just can't get enough.

are my kryptonite by Grimm Whiskey October 19, 2015