It is as simple as saying "see you later" while you wait and watch how "later" never comes. How many times you've said your byes to a person? how many times you've crossed a name out of your memory wishing it to be any other way, but the fact is, it is what it is - another soul sailing along your sea and not wanting to accompany you in your voyage to the shores.
The trick remains simple though, hard to apply but simple to comprehend and understand, people are lessons to be learned. Some teach us good traits and others teach us how to overcome our bad ones, very few impact us in ways they could never imagine possible. At the end, we all teach each other something. It depends on how well we receive these lessons and on our ability to learn from them.
As it is the case with life on the large scale, letting go of people is not an easy process. On the contrary, it is far from smooth and it is an ongoing battle of "shall I keep fighting to keep them around or shall I just let go and move on". I've learned that humans who are meant to stay in your life will stay - whether you do an effort or not - and humans who are meant to be stations in your life will be faded in the background - even if you fight the world to stay where they are!
The human mind can be tricky. It can trick you into thinking you are not worth it. It is all your fault why people are leaving you - but its really not! it is not your fault, or even if it is, it is not solely falls on you. Any relationship in the world contains two pillars (you and the other person) and it takes effort from both sides to make the boat sail, if one of you decides to let go or loosen up while your tightening your grip - the balance will fall short and the boat will rock and eventually sink.
Smooth sail and move on. The trick is in your recovery process. Trust the process. Trust your abilities to move on. Trust that the universe is there to get you the best out of this life, not the other way around.