It's a thing, you all, and we've got the antidote.
Acknowledge that even if it was one-sided, something did happen.
Tune out your friends who don't understand your heartbreak. In some cases, getting over someone you never dated can be harder than moving on from a relationship. The first step is to accept that what you feel/felt is real, and therefore, deserves healing.
What made you get attached to this person? Is this true about them?
When you crush or love someone you're not in a relationship with, it's not weird to base so much of your love around things you perceive. For example, if you saw them opening the door for someone on campus, it doesn't necessarily mean they're kind. Think about the qualities that you love most and ask yourself this: Are they real or did I make them up? What was attractive about them?
Listen to Olivia Rodrigo's SOUR album, or other albums like it.
Music heals, you guys. Don't overestimate the power that a good listen and a good cry can have on your healing process. I recommend Olivia Rodrigo's newest album because it has songs that speak to this specific topic, but anything similar works. Head phones on, world off, and feel it all.
Rip the band-aid, kill the fantasy.
Moving on from someone you never dated can be painful, especially if they're not aware of your feelings. Not everyone gets a closure, which can make the healing process much longer. So, while it might be painful at first, kill the fantasy. Don't re-read their texts. Stop flirting. Block them if that will stop you from seeing their insta stories over and over again. For you to heal, you need to burn the bridge.
Go out and kiss some strangers.
Or text them. Whatever you prefer. Again, since most of the time these kinds of break ups do not come with closure, getting out there can help. I bet you there's a high school crush that still thinks about your or a friend of a friend that would make a fun first date. Don't spend your time watching them live their life. You're not getting time back, so might as well move on to better things.