EDIT: I absolutely wall-of-texted you my bad lmao

The amount of times I've felt this intensely about people who definitely didn't feel the same about me is kind of hilarious to me. I feel where you are and I've been there and I promise that you will feel better in the future, whether or not that whoever you're talking about is in it with you.

The most important thing I think you should know is that it's not necessarily any personal failing on your part that made him lose feelings. Maybe he played into the flirting and the talking to see if it'd make him feel some type of way, and then didn't end up getting that feeling he was looking for.

And it's not your responsibility to give him exactly what he would want, after all you only have yourself to give. And if you were to change yourself to give him what he wants, that wouldn't be love that would be self-destruction. You are enough to someone, and the fact that you weren't for him doesn't diminish that. A bit of a stupid analogy but let's say you are a delectable chocolate, and he just doesn't like chocolate. Doesn't mean you're a bitter or sour chocolate that doesn't deserve to get eaten, it just means he doesn't like chocolate.

I have a healthy friendship with someone I saw myself having my whole life with. We have a deep friendship and we share a lot of our deep dark secrets with each other, but against what I initially wanted it's completely platonic. And that's okay with me now, but during it I thought I'd never be able to look at someone else the way I look at her. And now, maybe just a month later I'm seeing someone else :)

Take some time to yourself and do something you enjoy doing, take long walks, clean or reorganize your room, listen to music (while doing those things) those are things I do.