Letting go means accepting that they aren't the same person you met xx months or years ago. They aren't the person who you'd talk to for hours over skype every night, or watch stupid movies with.
It means looking at your phone every morning and seeing nothing, except maybe a text from your mom asking how your first day of classes went. Instead of seeing a text that reads something along the lines of something romantic, you feel like one of those girls in a forever alone meme. And your heart sinks every time. Letting go means going on that date with that new person, just to distract yourself from the pain of what you felt when your love left you. You know deep down rebounds don't work, but you do it anyway. You think "maybe this time will be different", and it ends up being the same every time. You'll do anything to not think about it. It means getting up and getting dressed every day, hoping maybe you won't feel the way you did the day before. It's a one step at a time process. Letting go means crying yourself to sleep more than a few times, and contemplating texting that person but stopping. You know it won't fix anything. But above all this, letting go means feeling a secret sense of relief knowing that this person wasn't right for you. Knowing that the time you spent "wasted" on the wrong person is over. You can breathe again. After all, what would have been worse? Spending the rest of your life with someone who you didn't belong with, or a temporary heartbreak? I'll take the second option any day.
Letting go means eventually accepting that they might find someone new today, or 5 months, or 5 years from now. And when you happen to stumble upon a picture on social media of them that would have crushed you only a few months ago, you smile and realize that they're happy. And you're perfectly okay with that. Let someone better find you.