Yeah, Trek, I was.
I have noticed that on some bikes it is much harder to stand than others, and that has to do with fit.
A case in point, when I first got my bianchi, since i was so stretched out there was NO WAY i was going to stand, it was too scary. After it was fitted to me
(handlebars up and in, essentially) suddenly I could stand a little.
Now here's the important part: standing requires you to be VERY strong.
As I get stronger, I can stand more. it's as simple as that.
If you Sara, are remembering being a kid and standing, you weighed 60 pounds and ran all day when you weren't on your bike. Today, you weigh more than that and your muscles are probably a lot softer.

So Standing is something for you to work towards, a goal.. And let us know how that goes.
And in the meantime, maybe get your bike fitted to you a little better?