Feeling worthless after easily being forgotten

I had an ex GF that dumped me in the late december 2016. Before that, we was happy together and had planned our engagement in August 2017 before she dumped me without giving any explanation. After sometimes, I can cope up with the grieves and depression until yesterday I found out that she got engaged to another man in February 2016, just several months after our break up. My heart was torn into pieces. Not because she got engaged to another man, to be honest I feel happy she has found a good handsome and smart man for her fiancee. It is the feeling that I was easily replaced and forgotten. I felt like I have no value as a human. You can imagine how happy I was when we talk about engagement but no more than 2months after break up, she got engage to another man. How can I cope up with my feeling of worthlessness? I feel like I just like an inconvenience trash for her. As soon as she felt uneasy with me, she dumped me and chose another person. I was still in grieves when she has happily got engaged. I felt like my grieves is for nothing. Please help me.