Fuel pressure problems

I’ve changed on my 2003 Silverado 1500hd the crank & cam sensor, ignition switch, starter, key cylinder switch, fuel relay, fuel pressure regulator and I checked the fuel pressure on it (which I did first). So it's not holding the prime pressure when I first turn the key but overall works perfect once I get it to turn on. So I gave in and took it to the mechanic and he said it was the fuel pump. So I changed it myself because I don't have $850 to pay him for 5 hoses, 1 electrical clip and 2 bolt removal. Guess what? It’s still doing the same thing; it will not hold the prime pressure needed to start the truck! Can someone help me out here! I’m about to drive the truck off the cliff! Well push it over if it doesn't start.