I've noticed that stitching decorative stitches over intersections where seams meet changes the size and shape of the stitch. My solution was to use the more simple stitches and to lengthen the stitch a little. Also--don't look too closely at your stitches--we tend to be overly critical of every little error. I know you want to do your best and learn correctly and that is good but I don't think perfect is a goal many of us achieve every time. Sometimes we have to accept and just keep working at getting better. So glad you are learning to quilt, it is a great trip even with a few bumps in the road.