Starting a sentence with 'If only ...' makes me cringe these days. Trusting my instincts is the best way to avoid that.
For every big choice we make there are a multitude of small, seemingly insignificant, ones. In the past I have tended to agonize over those big ones for a long time, but once my mind was made up, I didn't look back (well, not too much, anyway).
Small decisions are easily, almost haphazardly, made but they are often the most important in retrospect.
Driving a car provides such small important decisions. Drive quickly and you might get to your destination faster or you might be in an accident. Of course, if you drive slowly, you might just as likely cause or be part of an accident. Either way, an accident is the least likely consequence, yet the difference between being here or there affects what happens to you and others around you from that time onward.
Thinking about those sort of small decisions seems pointless - the consequences seem beyond our control. Yet, we do make those choices and our mood often plays a huge part in them. Anger might cause us to be aggressive in our choices. If we are distracted with other concerns we might drive slowly and carelessly. No matter what, we must pay attention to the road. Before driving it is wise to calm yourself - if you can. In fact, I think having a calm mind, as much as possible, is a key to success in general.
I believe that following your instincts when choosing a course of action tends to work out well more often than not. There are casually instinctive choices and insistent choices - feelings that I ought to do something in particular right now. When I am calm, not distracted by other things, there is an internal guidance system available, and it works through those feelings.
Ok, I know that sounds weird, believing in guidance in such small things, but when you think about it, if those small choices are really important, then trusting yourself, believing in some guided part inside, is a very good idea.
In Harry Potter and the Half-Blood Prince, Harry wins a vial of Felix Felicis a.k.a. Liquid Luck, a potion that gives the drinker good luck in all their endeavours. In the story, Harry pretends to put some in Ron's pumpkin juice just before heading out to play Quidditch. Ron plays a perfect game, but we find out that there was no potion in the juice. It was Ron's confidence that all his choices would be good ones that made him better.
Making choices, confident that you have done the best you can, might be the very best way to live. Even if/when things don't go as well as you hope, you are less likely to second-guess yourself. When things don't go my way, I can rationalize the way things went by thinking 'I did my best'. That way, I never have cause to start a sentence with 'If only ... '.