I know how you are feeling. My first civil war quilt was like this when I was trying to make 6 1/2 in sqs. My teacher checked my seams and they were 1/4 in. Then she shared a tip with me that really helped. I now use Aurifil thread. It may not seem like much it being a 2ply thread and most others being 3ply but when you are looking at several seems in a small block it can add up. Also, not all rulers are the same. Use one ruler for the whole quilt so you get consistancy. I also move my needle over so that it's slightly more than a scant 1/4 in so I'll have some room if I need it. I'd rather square it down then not have it big enough. I like to starch my fabric because it helps keep it from stretching and if I'm doing triangles, I really like to starch it.