Takes time mate, but id say start talking to other girls and you will forget soon enough.

Get back on the horse and you'll soon forget it.

It's a hard thought cycle to get out of, you could try sleeping earlier and setting a time to put your phone down, or you could confront it head on and speak to the girl

Me and my ex girl stayed together for 1 year and 7 months, it's been more than a year that we broke up now and I still think about her everyday
She was my first ever girlfriend so I guess i'll never forget her...

I recently start a new job a few months ago and started to talk about this girl thats 4 years older then me and is married. It sucks cause I think about her daily. I told her how I feel and stuff and she pretty much said, if she wasn't married she would be with me. So it's a waiting game right now as she may be married but are not happily married.

Time my friend.

Go talking to some others girls (preferably 2D girls) and take your mind of it.