Why won't the plug fit my coolant temp sending unit after I just p

[gm15004.8]

I just pulled this plug out today for my sending unit for the coolant temp and I cannot fit it back in for my life. I tried a billion times all different ways but i cant get it all the way in. It is very corroded but if I got it out I figured i'd be able to get it back in. This is on a 4.8L Vortec in a 2002 sierra 1500.