Swing open the damn door and embrace it!!!
That's all there really is to say on that matter, honestly.
I feel like multiple times opportunity has knocked on something of mine; a door, a window, my head and I've either looked the other way or ignored the sound all together. Reflecting on where missed opportunities has gotten me in life, has made realize that I allowed my fear of the unknown to override my desire to see myself succeed.
People don't miss opportunities on accident, it's something that they do on purpose; something like a choice if you will. You can CHOOSE to answer when opportunity knocks OR you can CHOOSE to ignore it. Either way, the decision is yours and yours alone. No one can make the decision for you; they can advise on the matter but they can't decide on the matter, because that is something that you have to decide on for yourself.
The moment that you realize that you have missed an opportunity and want to get it back, you must then go out and make an opportunity happen. Put yourself in the position to create an opportunity for yourself. Find doors and then kick them open. Take the bull by the horns and ride it to the ground until their isn't any buck in it left. Take charge of your situation(s) and make it work for you.
Every opportunity presented in life is meant to bring you that much closer to your goal. To your dream. To happiness. It's up to you to embrace these opportunities. When you embrace the opportunities that life presents, it will minimize the negative reflective moments that you will have about your past.
Life = Opportunities. Opportunities = Dreams. Dreams = Happiness. Happiness = Life.
It's just that simple....