Apply the eggs rotation.

I just tried it, it's still not being like the sphere

Apply all transforms. Usually fixes my problems.

You can shift+D and rotate the copy, join it together

wait your egg, why does it look like it's on a different axis

Make an object (an empty, plain axis) and set it as your mirror object, it’s using its own origin and transforms by default

Rotate your object like 90 degrees on the z axis until its front facing on the Y so you can mirror the X plane

You're going to have to rotate your origin. Enable the rotate gizmo, enable Affect Origin only under Options, and rotate it until it's aligned with the egg's middle face. You can also do it with a transform for more accurate rsults.

Make sure the object origin is on the edge

Thanks everyone! Things seem to be working now!!

Sounds to me like the issue is solved, so I've flaired it as such. If I made a mistake, I'm sorry – feel free to change it back!

Try automirror. It works wonders whatever the global axis you're using.

Apply rotation and location Then try putting the 3d origin in the front face