It should be obvious by now that Marines a very decisive. The real question is, how do we get that way and, more importantly, how can you get that way. Our success around the world comes from several things. Usually it is just pure guts. Secondarily are things like training, taking initiative, and making decisions. Our general theory on decision making is that any decision is better than no decision. MCDP 1 Warfighting states, "If we fail to make a decision out of lack of will, we have willingly surrendered the initiative to our foe". Surrendering the initiative is the last thing we want to do.
Remember that we orient on the enemy. All our decisions are based on how we can ruin his day. If you would like to improve your life then you need orient on your problems and impose your will on them. They typically will not enjoy that and go find someone else to bother. Like maybe the Army. But that's not my problem and it's not yours either.
The critical factor in decision making is time. The faster you can come to a sound or semi-sound decision, the better. This is how we seize and maintain the initiative. This is how we present the enemy with a constantly changing environment that he is ill equipped to handle. This is how we knock him out of the park like a steroid induced home run into "splash bay."
Quick decisions can only come from experience, training, and intelligence. Two of these things can be fixed by positive actions on your part. (If you weren't born intelligent, well you're going to have a hard life anyway so I don't know if I can help you.) Seek out opportunities to gain experience. Watch and learn from others who seem to make decisions easily. In other words, keep your eyes on the Marines.