Tips for evening out

Currently, I'm working my way up to running a 10k 3x a week, and doing ABC for 30 minutes 2x a week. I've been using a 25lb kettlebell, but that felt too light so i got a 18kg bell. Something I'm noticing, is i can pretty much go forever with the 25lb bell, but with the 18kg bell my left side goes out after 3-4 sets, while my right side can keep going.

My question is this: how can i even this out? Thanks in advance

For reference, I'm 28m, 6'1", and hovering around 165lbs