Well, I've been like this ever since I started riding, through 4 road bikes. In fact, the first few times I went to a spin class, I couldn't even stand up there. Now, I could stand all day on a spin bike, although I don't spin much anymore.
I can stand when my bike is on the trainer. Perhaps this has to do with my lack of balance. I always feel like I am going to fall over/crash when I stand. Both of my bikes have been fitted, one is custom and the other went through the laser/computer fitting. The custom bike has a more relaxed geometry, but both are very light weight (14.7 and 16 lbs. without the bags, rack, etc).
I've only done very very short stretches of anything over 15% grades, mostly in Austria and the Berkshires. The 10 mile (17K) climb i did in Spain just about made me puke, but I did it; it was a cat 2 climb on the Vuelta and the markings were all over the road. I just would like to be able to power up a hill for awhile, like my DH does.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport