I can say from personal experience that you can either use your past to propel you into becoming a better version of yourself, or let it sink you deeper into your sin because if you have done it once what is the harm in doing it again?
I dabbled in the bitter realm... and while I have done many things I was not proud of, I decided I could not let it ruin the rest of my decisions. You cannot quit life just because of a foolish decision one time. For me, it was about my younger brother and sisters above all. I wanted to be someone they could look up to. I wanted to challenge them to be better and not have them thinking in the back of their mind that I was a hypocrite who didn't know the first thing about worrying about others first.
Selfish tendencies are something we have to fight against our entire lives, but just because you have been selfish for the greater part of your life doesn't mean you need to be bitter and get more selfish as your time goes on. In fact, there is no excuse to ever hold on to something just because you are comfortable in it.
People far too often let their past predict their future, when it should not be that way.
Be encouraged today that it is never too late to change, and no matter what your past holds, your future is still determinable. Find hope in the fact that everyday is new and we are able to redefine ourselves as often as we feel led. Without growth, we would be bored.
Look into your life and allow yourself to see bitterness towards people and things. Make every effort to remove it, because with that comes peace of mind knowing that nothing stands between you and God.
Be encouraged dear friends. I can guarantee your journey is just beginning and that God will continue to mold and shape you and work your past into a glorious future, if you will let him.