Everyone here knows the typical advice for moving on when feeling blue about The One that Got Away:

-No Contact
-Lift
-GFTOW (Go Fuck Ten Other Women)
-Meditate
-Bro Time
-Approach (Which is actually easier during Oneitis)

But I'd like to share some little-known tactics that are often overlooked:

-Change Your Surroundings
Rearrange your furniture. Put up new wall hangings: posters, paintings, photos. If you're able to do so, move. If you browse your phone or computer a lot, change your wallpaper, sounds, and case/bag.

-Alter Your Soundscape
If you listen to music while driving or at home, try new genres, a podcast, or even silence. If you don't listen to music, try putting on white noise, or instrumental genres (classical, new age, electronic)

-Remove All Reminders
Anything that triggers a strong response must be destroyed or removed, as much as possible, as soon as possible. That mug she used to like? Toss it. The DVD you watched together? Give it away. Don't return her items, if she hasn't asked for them back in a week or less, they are unimportant to her- destroy them.

-Change the Very Air You Breathe
This was a big one for me. Every time I took a shower, I felt huge waves of relapse and longing. Eventually I figured out why: it was my shampoo. We took showers together often, and she used my soap and shampoo. Their scent ended up in her hair and on her body- and each morning I was bathing in its super-concentrated form. Rubbing it on my face- essentially torturing myself. Throw away your old bath products, start fresh with a new brand.

-New Hobbies and New Friends
Get out and try something new. A new sport, learn a language, travel. Stay at home if you were always out. Meet new people and eat new food.

That's all I can think of at the moment, but the principle is the same for all of it: be alert to opportunities to change your mindset via new input and environments.