Stop complicating your core training with non-stop crunches and leg lifts.
Start with basic lifts that build the foundation of your core.
Think about strengthening your core through compound strength exercises such as the squat, deadlift, bench press, and overhead press. To make your core stronger, you need to let it work when you put heavy loads in your hands or on your shoulders.
From this perspective, every exercise can and should be looked at as a core exercise. Consistently increase the load (weight) over time (with good form) and the core will naturally get stronger.
Standing exercises demand the most from your core. This makes the overhead press the king of all core exercises!