Alternatively, you can do it this way (see video below). Essentially what is happening when you do the mirror with the original options, it is merging the vertices together automatically. BUT it is unfortunately merging vertices that should not be merged because they are far away from each other. But since the threshold level is being set automatically, it is grabbing vertices far apart and merging them anyway. So by setting a custom threshold, you are forcing Maya to only merge vertices that are very close together, thereby preventing the model from melting together like it is for you.
Also at the end of this video I show you what Delete History does.
Please mark this or the other post (or both) as the solution if it works for you.