Q: No power when accelerating

I own a 5 speed manual fuel injected Kia Sephia. I bought it used and the check engine light was on. It ran perfectly,so I ignored the warning light. In a few months I felt a very small hiccup off and on when I accelerated. It wasn’t bad, so I let it go. After a month went by it started hesitating very noticeably when I was cruising with the gas pedal held at a steady speed. Then the hesitating got more and more frequent until it turned into bouts of heavy running of loud and slow - then it would run normally again. This happened off and on. I revved up the car to high speeds to keep it running at a normal pace because going slower caused it to hesitate. A few weeks later it did nothing but run loud, heavy and shaky when idling. It almost shut down when I pushed on the gas pedal. Revving it up to high speeds didn’t help it run normally anymore. Up until now when I start the car it runs heavy, shaky, loud and high revving does not good at all. I did replace the exhaust system, spark plugs, fuel filter, fuel pump, fuel pressure regulator, air pressure valve, air idle control. But none of these parts helped. I can no longer drive the Sephia because it has no power when accelerating, and the engine shakes and runs loud. I haven’t replaced the injectors, the injector wire harness, or the coils. Except I did replace the coils and the car still hesitated, so I took them back to the store. When my alternator belt broke I drove the car home. When I put a new alternator belt on all the problems returned. Can you please help me fix my car? Do you have any ideas what this could be? Thank you.