Nothing is going to be with you for a longer time in a constant manner. life is a journey of changes and we have to adapt to it. There are good times and there are bad times or rather not that good times. But nothing is same forever.
Spending a good time in a relationship gives you some awesome moments which you would love to keep alive forever. But what if those memories start obstructing your way ahead to adapt to the changes? Certainly, those good time spent together sometimes ago will perish with the struggle of adapting to what life has offered today. It was always a good time in the past, but if it is not your present today, it should not be a roadblock for future as well. Every relationship gives you a learning into several aspects of day to day life. It is also a test of your emotional intelligence. So, why not to carry forward that learning and make a stronger foundation for a new phase of life. Instead of forcing yourself to perish those good times, its better to carry forward and move towards improvisation.
People often loose a larger portion of their life in deciding what is good and bad, whether it should be done or not. Even then, life has much to offer. Doing things which gives us happiness even for a while is far better than repenting later on for things you did which you were not willing to.