I don't know if this is one of your first attempts at this pattern or not but some patterns are difficult. I think that happens to all of us at one time or another. That pattern, though simple is a lot of corner matching. There's great advice here. Pieces the same size, seams the same size, pinned with thin glass head pins all come into play. Give yourself a break, carry on and look at it as a practice piece. It will turn out fine.