So I just want to start this out by saying its not your fault. Do not blame all this on yourself.
Sweet girl, trust me, I understand what you are going through. I got my heart broken when I was fifteen by an older boy I had been dating for eleven months. I think most girls go through their first heartbreak around 14-19 years old. And you know what? That really freaking sucks because that is a major developmental time period for us in our life. So if you are still stuck on that dude from a month or even years ago, just know that its normal. Everyone still thinks about that one someone every once in a while and if you don't, you're lying to yourself. But now onto my advice.
Give yourself time to be sad
I know that you're probably going to want to go hoe around little bit because you see him with a new girl already and you want revenge and I understand that. However, that will mess you up more than you know in the long run because you wouldn't have really given yourself a chance to fully process everything that happened. Spend this time to cry, do a face mask, and just do all the self love things us girls love to do.
Lean on your support system
Talk to your friends, family, whoever you want to about it. Talking about it, will also help you process everything and understand it. My friends and family helped me out a ton. They would constantly check on me and help me out if I was having a bad day. They would make me laugh when I was sad, they would hangout with me whenever I needed human interaction. Even if I didn't want to, they would drag me out of bed and take me on some adventure to get my mind off of things.
Love yourself and get back out there
Guys have this way of indirectly making us hate ourselves when we did nothing wrong. So say screw that and learn to love yourself. And when you feel the time is right, get back out there. Don't let one stupid boy ruin the rest of your love life. There are so many good guys out there waiting to get noticed by you. Go find them.