Maybe you're just not a stander? I can stand, but am not comfortable doing so as it spikes my HR in a short period of time. I also produce less watts standing than I can seated. I stand mostly to change things up a bit. When climbing, I'll stand part way up the hill as long as I am able (around 80rpm) then sit in the same gear to finish over the top. Most of the time in the same gear, but if I go to long I need to drop a gear to rest a few revolutions before shifting back up a hill to finish.