I'm finding downtube gearshift a real shock to the system. My childhood bikes all came with Sturmey-Archer 3 speed - that little thumb lever worked fine.
Basically, how do you change gear yet remain alive? My current technique....
a) Find a straight stretch of road completely free of trash, pedestrians, buses, drain covers etc (in London this may take a while)
b) Stop pedalling
c) Hold handlebar stem with L hand
d) Lean back, standing on pedals (bike seems to balance better)
f) get both hands back on bars & give prayer of thanks
NB If gap between e) & f) above is more than about 1 msec, death seems assured.
The mantra that training on a single cog is good, is wearing thin. I guess there is a better way.