The only advice I can give you is what works for me. I sit very straight, my neck not bent, just lower my head to look down. I don't not slump my shoulders but keep them pulled back, in other words jut out my boobs, look down and sew. Have the machine lower then you think it needs to be. It should be like keyboard height. I can sew all day and not have any back, shoulder or neck pain. This is the way I sit. I sit this way in any chair now. Even the car.
Have you had your spine checked to see if it is out of alignment? That could be a big factor. I hope you find a solution, it's not fun to hurt.