I can't seem to break any blocks while in sneak mode
I'm using a Macbook and have already tried to use a different mouse, but got the same outcome. I've also tried changing the Game mode and still the same outcome occurs. For some reason, my left click just switches to place. This happens to every type of block as well.
Just a guess, but have you rebound your Sneak button from Shift to Control?
In MacOS, Control+click is translated to right-click so, if Control is your sneak button, anytime you're sneaking all your clicks will become right-clicks.