X2.
I too am a lefty and prefer the Glock most definitley.
As far as operation of the controls, no problem.
I use the meaty, middle part of my trigger finger to
activate the slide catch. (it pivots easy with moderate
pressure and catches every time). Keep your finger
straight, along the slide and the catch with contact the
underside portion of that finger. On one of my Glocks, I
have even bent the catch outwards, just a blonde one,
bringing it away from the frame a tad. It works too but
is not really needed.
The mag release is depressed with the tip of the same finger
easily enough and I don't have to break my grip.
Now, it may be because I have hands a bit on the smaller side,
but I have showed 'larger lefty handed' folk these methods
and they seemed to work for them also.
Something to try next time you're shoot'n a "G" model.