Every time I go to pump gas in 2009 ford mustang. Almost every gallon that pokes in to tank makes the nozzle trigger pop back like if it was full. It is annoying to hold it and still pops back out. Basically every gallon I pour in it gas pump trigger like when it fills up so gassing it takes longer than even if I hold it with my it still pops it out like if it was full.
When the car's fuel tank and associated plumbing are operating correctly, the tank will accept fuel as some reasonable rate out of the nozzle at the gas station; that's what you're used to. On the other hand, fuel nozzles (in the USA, anyway) are designed to shut off automatically when the nozzle senses back-pressure coming up out of the gas tank.
What you're experiencing is the nozzle shutting off the fuel flow into the tank because the tank is generating backpressure. Excessive backpressure results when something is broken: it might be a physical obstruction in the filler pipe, or the filler pipe might be partially crushed, or some part of the evaporative emissions system (hoses and one-way valves and such) is faulty.
You'll have to carefully inspect the system, and compare it to a stock, properly-operating one. The car's OBD system may also show a trouble code that'd give you a clue; many auto parts stores will connect their code readers for free and read any resulting codes.