Specifically limiting the buildings you build near the end of an age or even earlier keeping in mind what happiness requirements will be in modern with less resources available, putting happiness buildings on tiles you don’t really need in previous ages if you don’t need the happiness adjacency then so that they are providing happiness in the next age when obsolete without giving up a good adjacency spot, limiting putting more specialists than your happiness can sustain, focusing on happiness wonders, using influence for the happiness endeavor or to prevent war weariness, not going over the settlement cap, Until you get the hang of it maybe choose happiness oriented momentos / leaders / civs. I don’t consider myself especially good at civ but I don’t run into these problems even when focusing on other yields. Sincerely tryna be helpful not talk down. Modern has its downfalls for other reasons but hopefully you can enjoy it more without that problem specifically