It's easy to understand how letting go of material things from the past disentangles us from the past, clears out space, and makes it possible to welcome in the new.
We can also understand, conversely, how forever keeping things from the past for the sake of it, can keep us bound to the past and stuck—and often does.
Purging ourselves of material things that have served their purpose and are no longer needed, from time-to-time, is very important.
Though all the same reasoning applies to people, letting go of people can present as more of a challenge.
We form more acute emotional attachments and the thought of letting go of that special somebody becomes unbearable.
People (including you) come, play their part, and go. Some stay longer than others, some shorter. Welcome them as they come, bow to them as they go.
Though there is always a short-term sense of loss from parting ways, over time, you will no doubt see, how necessary it was and how much your growth depended on the letting go.