Look around your room right now. Look for an object that you keep there that remind you of a certain person or even a special moment in your life.
I am sure that as soon as you started staring at that object a specific memory started running through your mind. More specifically a special memory to you that reminds you of that moment in your life. You started remembering all the people hat there with you, maybe you can even remember other objects around, maybe the smell in the air. This is how our brain works.
This exact example I just described is exactly why we should not keep sentimental valuable objects from our past relationships.
Some of the best memories that we might have could be represented by a small stuff animal, maybe by a homecoming proposal poster. These things will damage your healing and detaching process from your ex-partner whether you believe it or not.
I get it, some of us are holders, I am one myself. However, nobody said that you needed to throw these things away.
Giving these objects to someone special and close to you and asking them to keep it for a while in the meantime that you heal should be enough. Once you believe you are better and all healed back, you might simply get those things back that remind you of that relationship.
Nothing lasts forever, not even the strongest of pain we might feel at a point in our lives. There will be one day where you will be able to see all of those objects and feel happy you made those memories with that person once.