Sorry you are frustrated, hope after you vented you feel better. I don't have any answers for you, but I know I have the same problem and for that reason I am afraid to work from a pattern. I know I won't cut or sew perfectly so up to this point in my life I have never used a pattern. Then when I mess up I just change my plan or add a square.... whatever I feel needs to be done. BUT then I will never have a show quilt with this technique I know.
I work on my quilts for fun and I don't want to stress over them.. at least not anymore.. there was a time I even got frustrated enough that I pitched the whole project in the trash.