I went they a very similar situation just a few years back. We had been together for about 8-9 years, had a kid together. I still wanted to try and make it work like you did but she wanted to go elsewhere.
The few things I can tell you is basically a lot of the stuff I wish I HAD'NT done. FIrst off, stop trying at all to make her stay. I know it seems impossible and you want her to stay so why would you stop but it doesnt help, in fact it makes it worse. She knows you want her to stay and she's still not. Theres no "different" way to tell her or anything you can do to make her stay. When a woman doesnt wanna stay dont try and force her, if her heat isnt in it its better that she goes. You may be able to convinve her short term to stay but trust me youre just prolonging the inevitable. Also, if she doesnt want to stay then why would you want her to? Dont get me wrong I tried like hell to get mine to stay but now that I look back at it I feel like a fucking idiot, like a desperate box and thats just what I looked like to her. Again, she knows you want to work it and she's declining and unfortunately theres nothing you can do about it.
If you still have feelings for her, which you do, then getting another "girlfriend" is a bad idea like the others have said. Just getting laid is cool but to get laid youre gonna have to hang out and go out with a chick and eventually youre probably gonna start talking to the chick about it and thats gonna just make the chick wanna leave. Its just too big of thing in your life and too much on your mind to NOT bring it up to your "date." The guy above who said dont mention it on your date is absolutely correct but the problem is at the point youre at (or at least when I was at your point) its kinda impossible NOT to bring it up cuz thats all thats on your mind. Youre really not even going out to meet a chick, youre just going out to get your mind off her so all another woman is gonna do is get your mind back on her!!! Sounds weird but thats how I felt. If you need to get laid or sucked off go to a massage parlor seriously. Spend $80, be pampered like a king, hell you can even tell the masseuse your ordeal but as big of an issue as this is right now in your life its gonna be hard to have any good "dates," at least it was for me for a while (like 18 months really).
This sounds dumb and like a shrink (a shrink told me first, among others) but the main thing you need to do is work on yourself and just feeling better. Try (and I stress the word "try" cuz it seems impossible) to just accept the fact she's gone. Not "its over for now" or "were taking some time off" , NO, its over and done with and the sooner you accept that the better you are. hanging on hoping will NOT help and even if your whole plan is to someday get her back this is your best course of action even for that as dumb as it sounds.
Whatever hobbies or friends you have or things you do that genuinely make you happy do those things often. Whenever you start to think about your ex literally try and think of something else, ANYTHING. It sounds dumb but the less you think about it slowly you start thinking about it less and less and constantly thinking about it totally paralyzes you from moving forward, and thats all you can do now.
I finally got to a point where I realized I had two choices, "I could get over it try to rebuild my life or I could roll over and die." Those really are the only two choices you have and I know you dont wanna just roll over and die, althoug at times it feels that may be best. Its tough man and dudes try to say "oh man its just a bitch who cares" but I;ll tell you when you really love someone and they leave it hurts man, and really bad. Nothing in my life has come even close to the pain I felt realizing the mother of my child was leaving me forever, it make you feel like a failure, that youre not worthy, and you'll never find anyone to replace her. But you will, but not as long as youre still hung up on her.
So really just try and accept the reality that she's gone and you have to start over, theres really nothing else you can do. Then as time goes by each day it will hurt less and less, some days will be worse than others but it does get better trust me but it takes time. Like I said it probably took me a good 18 months before I was legitimately "over it." Dont listen to the people who act like its no big thing, but dont let it ruin your life either. Again, just accept it and try to move on. One day (like I am now) you'll look back and regret even wasting as much time as you did worrying and trying to get her to stay.
Dont call her either, that just prolongs it too, you literally just need to act like she's dead in a sense, just non-existent. Plus, no woman is gonna come back to a man that acts like he cant live without her, that shit only works in the movies. Women in todays world see that as desperate unfortunately.
I hope some of this helped and youre in for a rough patch my friend but dont beat yourself up too bad over it and try and stay positive man. Unfortunately this is life and you will come out stronger on the other end of this. I hated hearing that at the time but its the truth.
Gl to you