My car was running perfect all at once it started missing really bad. So bad to the point my car shakes really bad. What could be causing this?

I would start by replacing the temperature sensor for the ECU. Sounds like it might be stuck in phase lock.

Have you checked each cylinder for spark,
I would run a quick compression test also.

The rocker arm has to be set perfectly so as to not be TOO TIGHT or TOO LOOSE.. TOO tight and it will miss/shake/vibrate ... too loose and it will TICK/make noise and not run well...

It is always your suggestion about that problem, what really causing is one of your High tension wires get open, check them one by one on ohm meter, you may not feel the problem causing that shake when you start the engine and run at its idle speed at normal, but running it 6mph the firing power goes a little bit down making the miss fire or poor combustion of one of your cylinder causing that shake, wheel or bushing problem may caused shake, but surely you will feel your steering swiveling left and right, that was the sign of unbalance tire or suspension component problem.

Struts

It sounds like there could be a miss. A miss is when the timing of one or more cylinders is off, causing it to run rough and sometimes shake badly. Im not positive it's a miss, but if the distributor or distributor cap is bad, it could cause it to start weird and run rough. It could also be the spark plugs and/or spark plug wires. There could also be carbon build up in the carburetor. I hope this helps. Sorry it's so vague, but without actually being there, its hard to say for sure.

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After a fill up, the fumes in the gas tank are very concentrated. If problem with evap system, it could cause problems after a fill-up. Just a thought.

it could be bad spark plug causing your engine to misfire

Generally a tune-up is needed when a vehcle is misfiring. Usually the tune-up is the least expensive way to go. It could be the fuel injection or relay, but tune-up is the cheapest way to start.

SOUND LIKE LEAKING HEAD GASKET.ANTIFREEZE IS BURNING WITH FUEL CAUSE SEVERE ENGINE MISFIRE THATS WHY ENGINE SHAKING BAD ITS MISFIRING.TIME YOU NEED A NEW ENGINE WHEN ITS SMOKES HEAVY BLUE SMOKE USE A QUART OF OIL ONCE OR TWICE A WEEK.