Here’s a hint - you don’t have much choice in this matter. It’s the nature of your mind to keep repeating your past events again and again. It’s a compulsion. And unless you develop a certain sense of awareness via mindfulness and start recognizing these compulsive patterns of your mind, you will find it very difficult to let go of it. Another reason why you are holding on to it is - because certain ‘happy memories’ provide you an illusion of happiness and comfort. When you are stuck in your happy past, you completely ignore your present circumstances, isn’t it? So, with this question, you need to ask yourself - why I’m holding on to something which isn’t my current reality? Whether good or bad, you can never go back into your past to change something or to live there forever.