The experiences we had in our past have created the person we are now. However, it does not define who you will be. Your life is an ever changing thing. It is not static, no matter how much it feels like it has stopped, it is still moving forward and changing.
Everything you do right now, is shaping the person you will be. It is constantly changing and developing. Try to imagine yourself as water, flowing with the current. You cannot go back, all you can do is go forward. You are pushed from different bodies of water. From small ponds, to rivers, rough waters, to vast seas. From each experience you encounter new organisms, and you go from feeling large, to feeling so small, entering calm waters to rough waters, but nothing ever is forever, everything is always moving.
This is how life works. As horrible as your past might be or as much as you may miss the memories with a certain loved one, holding on to the past is as stressful as holding on to a rock in a strong current pushing you the opposite direction. You will try everything in the world not to let go of the past, but sooner or later, you will have to let go, because something else is pulling you. Something is waiting for you.