I mostly play sitting down, but I try to play standing up every once in a while to get used to it. But it's like I'm having a problem with my left hand bending too much, trying to reach the E and A strings, or having to use my pinky and ring finger (I'm playing bass, if that helps any.) It's also hard for me to keep my wrist upright, and I end up bending it to the left a lot, with my thumb near the top of the neck + extending pretty far .

I'm also having problems playing anything above the 12th fret while standing up. I just can't reach anything when I'm playing down that low, and it begins to hurt my arm, wrist and everything else. Are there any posture advice or tips you can give?
wear your bass like a bib when standing up, just make sure your bass isnt too low down while you're playing
i don´t play bass but have you already tried to make the strap shorter so that the bass is higher while standing?
raise your bass up a bit so you can reach farther with your left hand and it allows your arm to be in a more comfortable position since if you play alot sitting down your arm probably go used to being in that position.
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