My ex is rubbing it in my face that he is dating someone else, why is he doing that?

Me and my ex broke up 20 days ago. I broke up with him because of his drinking problem. He was begging and begging for me to come back to him, but I said "If you are in a rehab then I will be with you, but if you continue to drink, then I won't be in your life." He kept trying and trying, and making compromises with me, I knew it wouldn't work unless he got the proper help.

So, while he is contacting my mother because I blocked him, he is confessing his undying love for me. I found out that he added a girl on his facebook that he was interested in. That was part of our break up. We broke up once before. He had this girl in his phone he was interested in. I told him if he wanted a clean slate then he had to get rid of her number. He did. He is single and can do what he wants, but don't talk to your ex trying to get back with your ex if you already have had sex or are interested in other women, or a woman. So, I got angry and said "I am going to date other guys. I am done with you. You have been trying to get back with me meanwhile you are talking to Ashley, and possibly screwed her? You are playing with my emotions. I am moving on." He didn't contact me at all. He contacted my mother and said "I met a girl last night her name is Alexandria. She's gorgeous and beautiful. Since Sara is dating other guys, I am going to date other women." Meanwhile I haven't even talked to a guy because I am so heartbroken. I was just mad and said that because he was toying with me. My mom said "Ok, you both moved on. Now leave it be. I don't want any parts of you." He then texted "Exactly." My mom didn't respond, then he randomly texted her saying "I am going to see Alexandria. See ya." There was no point of that? Why is he hurting me more. I just keep crying and crying. Why is he shoving this girl in my face? He went from pondering over me, to going out with other women in an instant. He keeps saying how he is glad the relationship ended.
My ex is rubbing it in my face that he is dating someone else, why is he doing that?
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