I had the same problem m8, trust me it will go back just will take a while to do. I reckon I have more cables than most behind my headunit, my 2 amps plus alternator thingy, aerial and cd changer wires, plus speaker wires.
I drove round for a few months with my head unit sticking out a bit, not very safe as it’s a Sony flip down and doesn’t come off plus my car didn’t have an alarm back then.
Anyway don’t bother with cutting the metal bit, just force the wires underneath that metal bit as much as possible, try to get as much of the wire under it as possible. Then force the headunit in as far as it will go, if it don’t go all the way take it back out move some off the wires and try again, trust m8 it will go back, I thought mine would never go back, and in the end it did, just keep trying.
Jonesy