Give me back my broken parts

I am thisclose to finishing my sister’s vest:

All that’s left is some seaming under the armholes (approximately 1.5″ under each arm) and ribbing around the neckline and the armholes.  Unfortunately for me, I can’t seem to get the hang of the mattress stitch (which makes an invisible side seam).  This is frustrating because I was easily able to graft the shoulder pieces together on my first attempt:

This seaming nonsense is driving me batty!  I’ll take this opportunity, though, to walk away from the project and clear my head.  Hopefully I’ll be more successful tomorrow.  In the meantime, I’ll continue reading Pedaling Revolution: How Cyclists are Changing American Cities.  (That book makes me pine something awful for a more bike-friendly Staten Island… and NYC in general.)