The glass is breaking because the area of the window is probably slightly smaller, for some reason. I would suggest taking the time to make a template of the window opening using a piece of hardboard and then take the glass to a glass shop to have it "trimmed" to fit. Otherwise it sounds like that the opening will continue to break any piece of glass that you try to fit in there.
A friend of mine went through 2 windows before checking for size. He had the glass slightly "trimmed" to fit into that particular opening.
Hope this helps.
Enjoy life to the fullest - in a VINTAGE VW