I understand, i really do. I'm 14 years down the line since a break up, and 10 years since last final contact, with someone who emotionally, financially and mentally sexually abused me,for over 2 years, and i have PTSD i'm still in therapy for. The kind of domestic abuse i suffered, (via online messaging, texts, phonecalls etc) became against the law as part of domestic abuse in May 2015, as opposed to physical abuse like hitting, and physical sexual assault, that was already against the law. BUT it only accepts acts of abuse from the date it became law as prosecuteable, so i can't get any justice.
In your case (i'm going off first post only) it doesn't sound like there was "abuse" as such though, more you simply fell in love with someone who love bombed you, then left you several months later after promising you the world. He'd very much have the counter arguement that at the time he said those things and made those promises he 100% meant them, but then fell out of love with you and, as he and everyone else has the right to do so for any reason, ended the relationship.
I sympathise that you are hurting, that you've had the rug pulled from under you of a future you'd mentally pictured and planned and you've had your heart broken losing the person you wanted to be in a relationship with, but there is often a "loser" when relationships end who didn't want the split to happen.
How would you propose that could be made illegal? That no relationship once started can ever be ended unless both people involved agree on splitting? How could that ever be enforced? That everyone who ever proposes, has to go through with getting married even if they fall out of love with their partner, because they "promised"? That divorce is abolished because you said vows that have legally bound you forever?
Would people who cheat be sent to prison? Who would decide what the line was to define cheating? Something physical has to have happened, or emotionally getting close and expressing desire to be with someone else?
If there has been actual domestic abuse, emotionally or physically such as coercive control, threats, violence, or sexual offences etc then that can be reported and investigated as that is against the law, but simply hurting someone by ending a relationship or not being completely honest with them, couldn't ever be made a crime as theres just no way to police that, and everyone should have the free will to walk away from a relationship they no longer want to be a part of.