I took a look at your save.
I am going to post a lot of criticism about what is wrong, but don't take it to hard. You are doing a lot of things right
I'll divide my post into general advice on your game and where to go from here.
Diplo: you need to be more carefull about it. Everyone hates you except Washington who is a nobody:
- Shaka hates you because you razed a city.
- Why did you declare on Monty?
- You traded with Alex' WE (ok if it was Washington)
Diplo is really ed up with everyone at war with each other. But the problem is that you do not have those lucrative intercontinental traderoutes because you only have open borders with Washington!
This leaves me to the next.
Civics: you are running the wrong civics. What you should be running is Rep, Bureau (you could argue for NH, but def not FS!), Slavery (you need this civic badly!), Free Market (and get open borders with the other continent for 5g TR), Free Rel.
You are in Merc which give +1 free Spec at the costly expense of no foreign TR. To justify this you have to make these free specs matter! All over your cities are running artist! and spies. It is a huge beaker waste. If you do want to run Merc take the time to change them to Scientists or Merchants.
City placement:
You killed 2 fishes in the SW

Dusseldorf is 1 from the coasr.
Hannover is fine, but it is not even working the silver?!
All in all placement is good. Especially your blocking cities in the north.
Workers: you have 8 and had to run Serfdom? Build more workers! 1.5 per city! Don't expand faster than you can build the workers to follow the settlers.
Trees are for chopping! At this point there should not be a single tree standing anywhere in your empire. When you get Math chop away. You need workers for this to - 1.5 per city!
You have build walls and even worse castles in some cities
Never build castles. They are among the worst buildings in the game. Never build walls - except in 1 border city on a hill facing Monty/Ragnar/Shaka/x. Washington is peaceful and won't DoW you. You would never need a single wall or castle on this map.
Your build order in your new cities is wrong, don't build those theatres. Every city needs a Granary and that's it. Some cities might need a forge or CH to, but other than that buildings suck.
Stop building Frigates and Musketmen.
What to do now?
1: Change civics (pref in a GA if possible with the GS coming in 3 turns).
2.: Get out of your wars. Get OB with someone over there. Possible gift Monty techs to get to cautious. Keep Monty and Alex at war with each other or bribe Monty on Darius/Cyrus (whoever the Persian is).
3: Build 1 more worker in each city. Bulldoze all cottages outside Berlin and replace them with farms/WS/WM. It is too late for cottages to mature anyway. Save the forests for LM. Get SP when possible.
4: Tech steel -> start building Cannons
5: Tech Rifling -> draft rifles
6: Take Washingtons 6 cities on YOUR continent.
7: at this point you dont need to worry about any AI. You can tech to space if you want or tech to Infantry+arty+destoryers+transport and cap the rest of the AIs. You wont need better units than Inf+Arty to finish any game.