How do I truly inhabit a character instead of just saying lines?

You can say the lines cleanly, and still something is missing, the sense that you have actually become the person. That gap between reciting and inhabiting is the whole art, and feeling it means your ear is good. You can tell the difference, which is the first thing you need. Inhabiting a character does not come from performing harder. It comes from knowing what they want in each moment and why, so deeply that the words become the natural thing to say rather than lines to deliver. When you truly know what a person is after, you stop acting and start pursuing it, and that is when it comes alive. Take one scene and forget the lines for a while. Ask what your character wants right now, from the other person, in this exact moment, and what happens if they do not get it. Play the wanting, not the words. The lines will follow, and they will finally sound like they are yours.