When to disconnect the A/P on approach in an airliner?
You are experiencing what real world pilots also experiences as they level up: unable to keep up with the plane.

You are unable to keep up with the plane because your mind is behind the plane. You are doing what you'd do if you're flying a Cessna: you react after you see. In a jet, things happen very fast. A large aircraft, like a B737, is also heavy and the controls are not as responsive. That is why it became impossible to you.

To fly high-performance aircraft, your mind has to stay ahead of the plane. Anticipate what will happen. Do not react to what is happening. If you only react to something that has happened while flying a jet, you're already too late.

Technique-wise, you should focus more on the attitude indicator (AI). The AI is the only instrument which tells you the future. Altitude tells you the present, and the vertical speed indicator tells you the past. These instruments let you verify if 2 seconds ago you did correctly. To predict what the plane is going to do in the next second, you need the attitude indicator. Say the plane is now pitched zero degrees, and it is descending too fast. First, make up your mind about the new pitch angle, say 2 degrees up. Then, look at the attitude indicator and adjust pitch to 2 degrees up. Only after that, you give the VSI a quick scan to verify your descend rate is indeed reduced. Do not adjust pitch by looking at the VSI. It will never work.

You may also be too ambitious by attempting to fly airliners in the sim before you have a solid piloting skill. Move up progressively. Fly on small planes first. Practice maneuvers, like steep turns. As your skill improves, slowly move up to faster and larger aircraft.