Are you able to adjust the height of your machine so that your elbows bend at 90%? This is the optimum position when you are at an office desk. You also need to be able to sit up and still see what you are doing, so not sure what compromises are needed to achieve this
Also, I believe that your machine should be quite near to the front of the table.
A break once in a while helps. In fact I went to one workshop where the tutor suggested standing with your back up against the wall every so often.
I hope that things improve for you.