Why do men shower you with interest and then walk away like it was nothing?

Li

Theres a guy, i didn't like him at first but wanted to give him a chance because he seemed to be really into me and would put in a lot of effort. I was a bit skeptical because i was worried he was going to be a fkboy. But after we went on several dates and he didn't try to kiss me or anything i started to think that maybe he's serious about me so i started investing feelings in him. As months go by and we continue our dates and get closer to each other, i start to worry as he doesn't seem to want to be in a relationship and has never talked about being exclusive and that kind of stuff. I know that sometimes no answer is my answer so i confronted him about it and said i don't think he wanted to be in a relationship with me which is what i have wanted and told him from the start. He comes up with excuses and says that i didn't show him much interest at first and therefore he lost interest in me and decided to just be casual with me and that he wasn't in the right place to be in a relationship. Fast forward to a few weeks ago I just found out that he was with another girl the whole time and my gut feeling was right and now it's been a year and a half later and they seem to be in a secure relationship together. Travelling and introducing her to his friends and bringing her to his events, everything i wanted back then with him. seeing and finding out the truth absolutely broke me. I don't know what i did was so wrong and why he didn't choose me. It makes me feel so unwanted and unworthy of love. I know i should just move on, but i can't seem to move past this horrible feeling which put me in a depression phase. I will never go back to him or ever contact him, but why does it hurt so much even though it has been a long time already... seeing another girl live out the dream i once had.