Hi - There are good reasons why it feels more difficult to play standing up if you always practice sitting down. Simply, the whole instrument sits differently when you stand to play, so it all feels different and goes against the muscle memories you have built up in your hands and arms whilst practising seated.

I also notice you say you can't see the fretboard; this suggests that you are relying on sight rather than touch to remember your way around.

There is little else to do to overcome this other than to practice standing up so you get used to the difference in position, and practice looking away from the fretboard so that you are engaging your senses of touch and hearing as well as sight.

I play in a band and we realised early on that if we planned to perform standing up then we needed to rehearse that way too.