I hear you! I feel your pain! And it is a real pain to quilt large projects (pain in the neck, pain in the shoulders, pain in the...).
I, too, am at the same stage -- working on my first large quilted project, and it is not looking very good. The squiggly lines that should be straight! The 2 inch borders that should be 1 1/2 inch! The skipped stitches! The scrunched up stitches that should be evenly spaced!
But, I hear in my head many of the comments of quilters on this board noting that it takes practice to learn this skill. So, I'm going to proceed and maybe a dozen quilts from now I will have learned the "secrets" to how to make my own process go easier.
Don't hate me if I am pleased to find another quilter who feels exactly the same as I do at this stage. Actually, your message has encouraged me. Wish I could give you a nice shoulder massage to encourage you.