[Stiglr]

#1: when making your iniitial half, make SURE that the inner most vertices are ALL right dead on the 0 line where the halves will join. That's half the battle right there. That's what's caused your gap in the first place.

Then, when you do the Mirror, select all the vertices on the seam, and do a Snap Vertice by distance. Set the distance to very low, so it will basically only snap together vertices that are within 0.0001 or something that small. Be sure your center vertices are all but right on top of each other.

Then, after it seems like the meld has happened, do an Optimize Vertices and Optimize Surfaces to make sure all is well. You might also try making the entire head 1-sided surfaces, to see if any surfaces are facing improperly or their vertices are improperly ordered by the Mirroring.