I know you have had a lot of encouragement - - this board is so good about that <g>
I haven't read every page, but the first thing I thought was: What didn't she think she had gotten?
I did see the part where you said your shoulders got stiff ... a lower table for your machine and lots of other tables to support the quilt will help you with that. One lady throws the excess bulk over her shoulder, but that is another story for another day.
I watched one of the tutorials that was referenced, (not all the way through) but, has anyone told you to make the needle go as fast as you can and then move the quilt at a regular, consistent speed?
First tendency is to only give the needle a little speed --- and I have a problem with the sound of the machine making my movements speed up, but I digress!
If you speed your needle up and slow your movements down, the stitches will become more consistent.
and, as almost everyone has said: Practice, practice, practice.
I am a lefty who has done fairly well with moving the pencil rather than the paper ...
and, something I am learning to appreciate from this board - - some quilters are more artistically motivated than I am. I am not an artist - - I do a few shapes, but my imagination is limited (to say the least) - - sometimes I will try something I saw someone else do, but usually just stick to simple forms <wave>