don’t worry about it, we were all there at some point, making crappy meshes!
you could go into the polygon sub-object level, turn on vertex snap, and build some polygons in between, bridging the gap. once the polygons are there, re-smooth it in your smoothing groups and it should blend right in.
in vertex sub-object, you could try turning on vertex snap, and right clicking on a selected vertex, choosing target weld, and dragging the vertex to the other vertex you want to weld it to. then, after you’ve welded everything you wanted, adjust them all to get the exact look you want.
if neither of these works, just say so, i’ll see what else i can think of.