Dry-fire practice helps, too. At least it helps with the the fine motor control it takes to hold a sight picture and press the trigger.
Two disadvantages for right-handers: One is that most of us are also right-eye dominant; so when we go strong-hand-only, our dominant eye is better aligned with the sights. Weak-hand-only, and we're typically having to do some aligngment adjustments that aren't a mirror image of what we do on the strong side.
The second is that the rifling in most barrels spins the bullet counterclockwise from your view at the breach. This means the recoil will not only be up, but a little to your right, too, as the barrel experiences the "equal and opposite reaction" force of the spin. With the gun in the right hand, we don't notice it that much because that rightward pressure is into the meat of the hand: you have your whole palm behind that side of the gun. In the left hand, you're likely to really notice it because the shot feels alien to start with, your left hand is probably weaker, and the rightward force is now trying to pull the gun out of your fingers, not press it against your palm.
TX has the real solution. Practice. Which I don't do much of left-handed.
He also mentioned canting the gun a bit. Tilting the barrel to the right just a bit, in the direction of your dominant eye, can help with both sight alignment and the recoil issue. Or at least so I'm told. The most effective way for me to shoot weak-hand is to quickly shift the pistol back to my right hand...