Hiya Deanna

What the others are saying about changing up one or two cogs at the back is right. Because you have more force when you stand, you need more resistance when you stand or the pedals can feel like they are scooting away from you.

The other thing that helped me feel in control when I was learning to stand, was being told to "hold" the nose of the saddle between my thighs. You don't actually apply pressure with your thighs, but you keep your legs positioned so that you know where the saddle is, and by default, the rest of the bike.

Good luck on your trainer, thats a good place to practice.