I can understand how you're unhappy about not being seated if you normally ride in an upright position with a lot of weight on your saddle and rarely stand on your pedals. The feeling of having your weight on your pedals is quite different from sitting, especially if you have little weight on your hands.
I agree that practicing scooting along on one foot especially sounds like a good exercise. The bike will feel "swingy" and wobbly at first at low speed until you get used to steering more with your feet and hands.
I'd also suggest practicing lifting your weight up off your saddle while riding or coasting. Don't "lose" the saddle completely, keep some pressure against your thigh (this is easier if you lean the bike slightly towards one thigh instead of trying to balance it dead centre). This should force you to start balancing more with your feet and hands. When you're comfortable just standing, try moving the bike around a little, riding gentle curves while coasting downhill.
I'm not an instructor or anything though, but from rock climbing I know that complex movements involving balance have to be broken down into small steps. And the older you are, the more time you have to spend settling each step.
I don't think you have a mental block, you just have no muscle memory of how to do it.
I spent forever underwater teaching myself the "back-bend" necessary to dive backwards into the pool. And then one day I just up and did it.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett