how to deal with a breakup?

my ex broke up with me on the 19th, the day before he broke up with me he told me about a girl at his job who liked him, i was of course jealous and it kinda bothered me because i didnt trust him and he told me he would try to distant from her and it pissed him off that everyone was mking him talk to her. He broke up with me saying he wanted to work on himself and because it wasnt working out, he later confessed that he broke up with me because he lost feelings for me which made me overthink because he was telling me how much he loved me before he broke up with me. i later blocked him on everything. On the 23rd he texted me saying he had a gf already and i have no idea who it is but i feel like its the girl from his job, which he probably lied about wanting to not talk to her(he lied alot in our relationship). Today at 12:52AM he texted me on Messenger( i did not have him friended) and he told me he fucked his gf, i replied with "did i have to know this?" and he called me two times and i typed "why do you keep calling me?" he blocked me before it could send. My friend George called him and my ex answered and said "who is this?" Geoege said it was him and my ex hung up. My friend called again and his new gf answered and said "this is his older sister" and my friend said "Well your alleged brother texted his ex saying he fucked his gf" and she was quiet and then hung up. i was shaking and very upset because its like he became a completely person. I texted his sister and told her "i know its weird to text you and have no reason to be involved in this because its a stupid situtuion but could you tell ##### to leave me alone? i don't wanna know every detail about him and his new gf because its weird and seems obsessive. its like he turned into a completely different person." i told her i was sorry for bothering her and she replied with "its ok and ye ill tell him" Honestly its very immature of him to do this to me when he clearly knows im tryna get over him:/