Use a thicker cushion to raise your hips a bit more, and lower your knees. When your knees are below your hips, it automatically tips your posture to make a straight back more comfortable while still being ergonomic. Most modern seats are designed the other way, with your hips lower than your knees, which contributes to back pain and poor posture. You might also be trying to straighten beyond the natural position of your spine. Imagine a strong attached at the crown of your head and pulling up gently. You shouldn't feel any tension or tightness in the neck or spine when you do this, and if you do you've gone too far. Try rolling your shoulders back slightly so the front of you shoulders are in line with your clavicle rather than a curve. Also remember that it's not normal to sit bolt upright all the time, and allow yourself time to slouch or sit in a relaxed pose so your muscles and tendons get to release instead of staying firm all day.