I find it so much easier to play guitar when I’m sitting down, but when I’m standing it becomes so much harder.
When you sit and play, are you by chance looking at your left hand while you play? If you have been doing that, then you are having difficulty watching your hand when you stand, which is probably why you are struggling. It just takes lots of practice to get used to not watching yourself as you play. Keep at it, you will get there.