I feel like I wasn’t good enough.

Short answer: find something else to occupy yourself. Being alone with your wounds and nothing to do will just lead to obsessing about it.
More in depth though, most break ups feel sudden from the perspective of the person left behind. It was probably a process over a fairly long period. It isn't that you're not good enough, it's just that people change, and especially with a first partner that means you won't always match up as well as you might like.
It isn't that he wants someone better, just something different than what you had together.

Awww *hugs :) guys don't think with their brains we think with something else. Unfortunately heart never breaks even :( it's not your fault so you should not feel like your not good enough for him your perfect just the way you are you just gotta find the guy that feels that way and sees that in you :) you just need things to take your mind off him hang with your friends watch something funny online see that you can be happy without him oh and smile even if you don't feel like it :) if you done all those and still find nothing to do message me if you want to talk :)

Consider that some people have issues coping with break ups. He might just seem to be moving on quickly because he didn't know how to be alone. Most young people don't have long relationships that last for over a year anymore, so he was very comfortable having someone, now he's just putting someone else in your place. But that other person will never be you. Try not to let his actions affect you now. He's not your responsibility anymore. Try to focus on keeping yourself busy and making yourself happy. I was in a 5 year relationship with my high school sweetheart, from 9th grade to after graduation. We even got married and had our own place together for almost a year. It didn't work out, we were unhappy. So now I'm 21 and divorced. Things happen and at first it will be painful. But once you try to accept the situation and fight through it and be strong. Everything will be okay ;)

I don't know the answer to that myself but I just wanted to say I'm really sorry that this is making you feel so depressed and I hope you feel better.