Firearms instructor for over 30 years and also a lefty. As a lefty I always felt the mag release and slidestop/slide catch were in a great place for my index finger.
Index finger is probably the most dexterous finger. It's hard to instruct online but just loosen your grip and fold or turn your pistol in your hand and them hit the mag release , insert mag as index finger goes to slide stop, hit slide stop as you reacquire your firing grip.
One problem with using your middle finger, especially in a non-static shooting environment. Place your pistol in your hand, situate it as you would to preform your reload using the middle finger. Now look at what you have supporting your pistol, only your ring finger and your little finger. Not much if you are moving, bump into something or even fumble the mag insertion, it is very easy drop your gun. This is only exaggerated with doublestack pistols. I had other instructors argue the fact. Had them place a dummy gun in their hand to reload, Slight tap on the end of the barrel as they reloaded and the gun would spin right out of their hand.
Try using your index finger, just give it some practice it is a much better technique. Or find one of the newer pistols where the mag release can be reversed, so you can use your thumb and has an ambi slidestop.