I looked at your save.
You are researching military tradition after assembly line and railroad. It must not be cavs (15str), but west point you are after. Then again, it doesn't appear that you have waged enough war to have a unit of sufficient experience level. I would research assembly line for infantry, then psychics (mebe biology), electricity, and industrialism (tanks for the win).
You need to switch to representation. Also, you need pacifism, caste system, and a religion. Two problems with this:
1) your defense is dismal; spend your huge cash storage to upgrade all troops. Note: I thought this before looking at other civs.
2) Nara does not have confucianism. It is too costly to change civic to organized religion to get a missionary over there (you have researched scientific method - obsoleting monestaries, can't build one).
I would avoid specialists in Nara and run as many scientists as possible in your other 3 large population cities.
The switch to representation, pacifism, and caste system (max scientists) should get you physics, electricity, and industrialism in a timely manner.
Well, I took a look at what the AI civs had, in regards to troops. You could probably research democracy, switch to universal sufferage, then build cannons in all cities, buying a few every turn.
Or, research physics then artillery and explode.
On further thought, I think you can upgrade all units, build cannons in all cities, leave 1 unit in each city, and
crush the world, right now.
About unhappiness (you did split the floodplains)... you are a 22 population. Stop that silliness and you will not have such problems

Try some slavery/whipping.
You need to attack more, the military advantage you enjoy could have been leveraged a long time ago. Have you looked at the power graph?
You need to focus on a victory and pursue it. I can tell from how far you have gone in this game that you understand the basics. Read for a couple days on this site, and no more settler level.
PS. Conquest victories rule domination. Domination is too easily influenced by diplomacy (vassals, etc), and culture (landmass). You can conquest this game easily. Burn, baby burn!