A question I get a lot is "how should I sit at work?"
What if I told you...
It doesn't matter.
"Your best posture is your next posture"
What I mean by this is whatever posture you feel comfortable in is fine and as long as you move between postures and get up and move once in a while you'll be dealing with less pains.
Being scared into sitting bolt upright all day potentially could be causing more issues for you than you think.
What could be more important for you is what you do outside of your work hours.
Get moving, start strengthening the opposing muscles you use when working and watch your aches and pains start to disappear.
Let me know if this puts your mind at ease when it comes to how you should sit at work.