Since you have already ordered the parts you are on the right track. The most likely cause of the problem is the tip of the trigger (sear) rounding its edges. The next most likely is the hammer full cock notch wearing. The least likely problem is the trigger spring, although anything can fail. It is also possible for the hole in the receiver that the firing pin extension rides in to ware but that would take a LOT of rounds. If you do not know what you are doing, do not have the proper stones or hones for the job and do not have a sear jig your chances of making things worse out weight your chances of making things better. You have not provided any good CLEAR photos of any of these parts so any further guesses are just that. SWAGS.