I've been sewing since I was 12 and have made items ranging from designer outerwear to business suits to simple and complicated quilt patterns. Just took a look at the seams I sewed for my latest D9P and there is not a single perfectly-straight line anywhere. Off by two threads here and there in either direction, or starting at exactly 1/4" and ending up who knows where. Don't blame yourself and just compensate with block placement and squaring up.