Failed relationship

Hi all, just wanted to share this little story. I’ve been with my now ex boyfriend for almost 8 years on and off because I believe he has commitment issues or honestly I don’t know. He’s always been the one to break up with me and then right away gets with other girls, starts sleeping around and partying. But then, he comes looking for me apologizing and asking for another chance. I know I shouldn’t have, but I’ve given him many chances because I thought that he could change. When ever we would go back together, he worked so hard to make it work but then once again, the same thing would happen. This time around I gave him a last chance as long as he would take therapy because of this pattern that he has of doing this, he took therapy for about 5 months and it was great. For 11 months, I saw a real change in him, I was so happy, we were doing so good. But two days ago I come to find out that it took one night working at a nightclub as security, for a girl to go up to him and ask for his number, for him to throw it all away. He was texting her and wanted to go on a date with her. I had to leave this time no matter how much it hurt. I just don’t understand why he threw all his effort down the drain for a girl he doesn’t even know. Has anyone gone through this, or could know why he couldn’t change? I felt his love, I saw it. But I don’t understand why he did this