Sadly this is very common (virtually universal).
I suggest finding high-resolution photos of the plug and socket online. You will see that there are tiny bumps that act as clips that hold it in.
Turn the PC off, unplug the mains power, press the PC power on button a couple of times (this will discharge most of the power in the PSU capacitors), then carefully use a pointy knife and release the bumps, make sure that you don't do any harm to the motherboard or yourself, perhaps something like a cocktail stick would work, perhaps even fingernails, but you really need to see the pictures to figure out what to do.
This is the only way I know of to do this, it's not your fault, it is simply a 💩 design from the USB design people. Some...