if you want them back
then tell them that you love them, that you are willing to work on things and on yourself. Tell them once, and then leave them alone.
People usually don’t break up without reasons. It’s a tough decision and a loss for them too.
Let them know how you feel and what you want. Communication is key in a relationship, it’s also key while and shortly after the breakup. If there’s an actual connection between the two of you, you will be able to work it out.
Don’t listen to some dudes on the internet who sell you an one fits all solution, recommending everyone to go no contact instantly. If there’s still some love, your ex will appreciate your openness and vulnerability. Complete silence may feels like you don’t care and it was the right decision.
Of course don’t chase, don’t act crazy, don’t beg. But be honest about your feelings in a respectful and self-respectful way.