I'm far from a good player so I too have problems playing in positions I don't practice much. playing while standing is a bit different and you need to get used to it, so the best way to do it, is actually playing while you're standing.

a few month ago I watched a blues guitarist at his performance and he held his guitar almost flat on his lap in a position that would make my classical guitar teacher cry like a baby. That position would be impossible to achive with a strap on while standing. Still the guy gave an excellent concert.

I think its always good to learn to play in different styles and positions but until you there play the way you're confortable. So you can also try the other solution to the problem and bring a chair ;)