If you have trouble loading on an open slide, there is no hope you can load a full mag on a closed slide. I don't understand why you are having trouble loading on a open slide unless you are holding things very oddly. I have seen students try to load at full arms length, but the gun and magazine should be only a foot or so in front of your face, like holding a book to read it. Push the magazine sideways and up into the gun with the heel of your weak hand, and push the gun against the motion. Should go right in.
Example
https://www.bing.com/videos/search?q...3&&FORM=VDRVRV