How I Learned It:

People come into your life for a season or for a reason. And sometimes its both. But the possibly the hardest part about any friendship/relationship is letting go of that person when the time comes. You may not want them to go but you cannot hold onto anyone who has already checked out. Sometimes you have to let it/them go for your sake (not theirs).
As tacky as it sounds, it maybe easier for you to let go of someone by blocking them on social media, phone numbers and deleting pictures from your phone. It sounds heartless and like your throwing away time but the trials and lessons learned from that person will not fade. I have personally made this leap and it helps greatly.