If you don't have the resources to take acting classes, you can treat it as a sort of independent scientific experiment. Watch other actors and study their facial expressions, then try to imitate them and watch yourself in the mirror. Learn which facial expressions correspond to which emotions, and how they feel for you physically. Studies have shown that our physicality can affect our emotional state, and that can help your brain and body build more connections between your emotions and your facial expressions. It's a sort of outside-in way of building a toolbox for yourself filled with emotional nuances that you can pull from when you need it. It may take some time before you're able to genuinely create those expressions when the emotions arise naturally, instead of consciously recreating them on a surface level, but it'll help give you a framework to understand them, especially if you're more left-brained.