1996 Chevy S10 Blazer 2 Door 4.3 vortec Auto trans 200,000 miles

I Have a 1996 Chevy S10 Blazer 4.3 vortec auto trans 200,000 miles Have replaced everything from fuel pump to Distributor, Cap, rotor, wires, New Fuel injection assembly, New cat, 02 sensors, Spark Plugs, new muffler, IAC valve, New EGR, New TPS. My Question is when I’m driving this truck you have to push the gas pedal down to get it moving, it acts like it don’t want to go and it is getting really bad gas mileage and when the engine is under load like climbing a hill the check engine MIL light comes on and it’s the same code every time p0302. Can you give any advice as to what could be causing this problem. Thank you very much for any help.