Here's some options:
One, buy a stronger spring.
I'm not sure about the size of the Mustang spring, but it's possible it was a standard 1911 spring.
Brownell's carry springs for both so you might contact them for interchangeability info, OR for a stronger replacement specifically for the Mustang.
Second option is to buy a new magazine catch, on the chance yours is defective.
It does happen that a factory catch is slightly under spec and allows the magazine to release too easy.