Some people think that when you hire a personal concierge or a professional organiser, you will need to throw everything away. This way of thinking can make people put off sorting and organising but it should not be the case.
When you are ready to start getting organised, you don't need to take drastic measures to make a difference. The simple act of giving items a place is an easy way to start and will naturally create more space. As I go through things with clients, some get donated or thrown away, but there are other things that my clients are uncertain about. That particular item may have sentimental value which gives the owner a strong feeling of attachment, making it hard to let go of. It's OK to keep these things.
There are other options to keep the memory alive (like taking a photo). But for the items we feel very strongly about, I say keep it! Group it with other really important sentimental items and give them a place so you always know where they are. Sometimes, we just need to hold on to items for a bit longer than others and it's often something we cannot explain. One day in the future, you will be going through that box and see that item and think, why did I ever keep that? That's the time when you can let it go.