I sympathize with the post-partum roller coaster of emotions and whatnot (I also have three).  But as a person with a common name, I think it's great that you gave Matthias a different name.  We're not an international family but I still insisted that none of my children would ever be called first name last initial, which is how I went through my life (Kristen W.).  There were too many Kristens for me to just be.  I always had to be further delineated and hated it.  Not a one of my children ever faces that in class.  Maybe they'll hate their unusual names some day but naming rights were mine so they just have to live with it as I did with the reverse. ;-)  Oh, and as an adult now, there are no fewer than 6 Kristens in my neighborhood alone so even moving around the country has not spared me from being one of the herd.

So there, reassurance from a perfect stranger.  That ought to help, right? ;-)
