As you do not indicate what you mean by "stops too fast" etc., it is impossible to advise what sensor to use.
Modern cars with air bags have such acceleration sensors installed. But it is not allowed to touch the safety circuit. You can build your acceleration sensor from a small loudspeaker by gluing a weight on membrane center. Then an acceleration of such device generates several mV that can be amplified and used to trigger an indicator. Use a higher-impedance speaker to generate more voltage.
You can also use a piezo-electric sensor, like a sound buzzer plate from a watch. Again, adding a weight will make it an acceleration sensor, and the generated voltage can be up to 1 V but the ceramic plate is fragile.