The first few things I see.
1. Build a couple boats and get out there exploring. You don't really know enough of the land to make good calls about when/where to attack.
2. You seem to be going Replaceable Parts/Rifling. Go for steel instead. Cannons will get you more wins with less casualties. Any unit can be left behind to hold a city afterwards. Use trebuchets for city attacks until then. They blow cannons away.
3. Run a religious civic, any work but I'm going to say Theocracy since you're in a war.
4. You're in 2 wars (once of which you're outmatched in), make more units. You have cities doing infrastructure right now when they can be contributing to a victory. When you fight, use everything you've got.
5. Unhappy, you're running slavery. Why are you suffering from happiness issues? Start whipping away.
6. More Trebuchets (until cannons). Personally, I run about 75% of my army as siege units. It's really unbalanced just how powerful they are.
7. Ignore Sitting Bull, Bismark has airships. You need siege weapons and you need to take one of his cities. That way you can trade it back to him for peace (if you go for Dusseldorf you can get horses for a few turns as well, knights will mess up his units in your territory... after siege wreck them). Alternatively you can crush his stack and take several of his cities.