The bad news is, it sounds like you're playing in a very un-ergonomic way when you're sitting that's really bad for your wrists, fingers, neck, and back.
Sitting slumped forward and twisting your head around to see the front of your fretboard and fingers is terrible posture, and your back and wrists will thank you the sooner you stop doing that. And tilting the face of the bass up towards your face is just as bad, especially for your left wrist than now has to twist all the way around the neck while you play.
The good news is, the position of the bass when you stand is in a much better spot ergonomically than how it sounds like you're holding the bass while sitting. And the other good news is, you don't need to see your fingers on the face of the fretboard. You can keep an eye on the side-dots and edges of the frets and you'll be able to tell where you are. That paired with your ears telling you if the note is right or wrong...
All it takes is a little consistent practice and you'll be able to play without having to stare at the front of the fretboard while you play.
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Here's a POV of how i hold my bass sitting. (The angle is a little exaggerated and my playing hand is a bit twisted because it's tricky to hold the bass and take the pic with my phone right up in my face.)
Having the bass about 30 to 45 degrees out from straight-across is much better for your left wrist, your right hand & shoulder, and your back. Notice you can't see the face of the fretboard. But you don't need to. Just practice without looking at it and you'll have it down in no time.
You can still look at your hand with respect to the side dots and fret-ends. And eve that is better because now the neck is further out into your field of vision than it is with it straight across your body, so you have to look down less and crank your neck less. Your hand and bass neck are always in your peripheral field.
Also, check these videos of how to properly and comfortably hold your bass.