advice pls

He broke up with me about 3 months ago because he lost feelings after a year of being together. For the first month he wrote to me every now and then and wanted us to try again. At the same time, he was already getting to know his new current girlfriend. I don't understand why he did that when he cried until recently and said that he would like this relationship further than me, but he can't love me any longer despite the fact that I'm perfect. Now I see them walking together, he puts her on Instagram all the time (he never added me only when I asked him) and I can't do it because he literally destroyed me with his behavior. How could he replace me so quickly? He said during the breakup that I would always be the most important to him no matter what happens and then he didn't even make me birthday wishes, so soon after I blocked him to go further and for a while it helped me and now it's tragic again. They started talking to each other literally 8 days after our breakup. I'm sorry if what I wrote is chaotic, but I have a lot of emotions now and I don't know what to do, I can't go any further.