My mig has two removable parts. The air shield and the smaller copper tip inside of it. Both can get slag and corrosion around them or on the threads, but both should unscrew with a pliers.

I'm surprised you never had the wire burn to the tip before, necessitating the tip's removal earlier in life.

Here's one tip I learned years ago. The shielding tip is often 1/2" longer than the welding tip itself. I trimmed my shield back so it's only 1/4" beyond the end of the welding tip. This allows you to get closer to the work and inside corners for better penetration. It also lets slag into the shielded area easier, so keep a screwdriver handy to clean out the slag often, or use anti-slag spray.

I use Never Seize on the threads of the gas shield, as well.

There's alot to learn about welding and if you have a chance, take a course at a local community college. I was welding for 20 years before I went to take a short course and was surprised at all the technical stuff I didn't know.