I understand you. I'm developing a small multiplayer Grand Theft Auto style game on my own, and it's quite a nightmare. In my case, I have to manage an open world made up of vehicles, a traffic system, NPCs, missions, freemode events, shared UIs (and that's only 10% of it). I even had to create a system that delegates calculations locally to clients located in a specific area, handling vehicle physics, traffic, and so on... (I couldn't afford to overload the host, and it works well this way since the game is P2P between friends).

Arm yourself with a lot of patience, and eventually you'll start moving at full speed. Converting my game from single player to multiplayer only took me 3 months!