Acceleration causes inertial force, and it is not relative, but why?
What is the reason for feeling inertial force by acceleration?
Inertial forces, also known as pseudo-forces or fictitious forces, appear in accelerating reference frames; the inertial obsever sees their action - centrifugal forces, Coriolis forces, Euler forces, as well as responses to simple acceleration.
The pseudo-force laws all appear in the form of Newton's Second Law of Motion, F=ma.
The occupant of the spinning teacup feels the centrifugal force! It is prportional to their mass. They also respond to the sudden stopping of a car, hence the utility of air bags.
D'Alembert was the first to devote extended effort to analyze inertial forces. His results paved the way for analytical mechanics.
Einstein, while working on the puzzle of how to fit gravitation into the framework of Special Relativity, noted that Newton's Universal Law of Gravitation was of the same D'Alembertian form, decided that gravitation must also be a pseudo-force, starting him on the long path to General Relativity.
So, in conclusion, pseudo-forces always appear linear in the mass and the acceleration. As Isaac Newton noted in his famous whirling bucket experiment, acceleration is always absolute with respect to every inertial reference frame.
You don't feel "inertial force". What you feel is the friction of the floor on your shoes, or the side of your car pressing against your body. These are perfectly real forces caused by the non-uniform motion of something in your environment interacting with you as your inertia tries to keep you in uniform motion.