You are probably holding yourself up with the wrong muscles--superficial instead of core muscles, the rectus abdominis (the six-pack superficial ab muscles) and the erector spinae (outermost muscles extending in long strips along the spine). These muscles cannot hold you up, as they aren't postural muscles; once you tire your posture will collapse.
Here's an easy way to engage your core muscles, which can hold you upright in a much more relaxed and grounded way.
Think about the two bones in your arm. The ulna extends from little finger to elbow. The radius extends from thumb up the inside of the arm. Our tendency, sitting all day and working at a computer, is to forget about the ulnar side of the arm and to act as if the whole arm is just one radius, so we rotate that around and get carpal tunnel pinching. The same problem is behind tennis elbow, climber's elbow, etc.
The ulnar side of the arm connects you to your core. So think about reaching your elbows down and out, like your elbows are wingtips and you're lifting them up. Feeling the stretch that goes right to the muscles between your shoulder blades.
Next, place your feet flat on the floor and feel your toes on the ground. Push the ground away with your toes. This activates core muscles in your low back (multifidi) and in the front (transversus abdominis, the extensor in the front of abdomen).
It should feel easy to sit straight.
You don't have to keep it up all day long. But when you remember, push with the toes and reach with the elbows, as a nice reminder of what it feels like to be connected.