To gain your ex back means to be honest, be respectful and allow time for change. Think about your mistakes and changes of habit. Focus on your communication and improve if possible without pressure. Ask if they want to talk honestly and respectfully. Then really listen! Use emotional reflexivity and share empathy. Focus on yourself: self-esteem, interests, life goals, etc. and make it a habit to be patient, and by your consistent actions show that you are different. If the relationship is not restore, then let it be and be okay with realizing that you must respect their choice to not be with you. But no matter what, take care of your emotional and mental health.