Dear SD,
Hello fellow human, you are currently going through a heartbreak, this is a natural phenomena that happens in the life of most people who walk this earth. It can be thee most emotionally traumatic experience while at the same time the most eye opening thing that could ever happen to you. You're smart to come to the reddit community. Let me try to give you some advice.
It's completely natural and very very healthy to feel sad, depressed, and alone. This is your mind reacting to the situation. When such a disconnection or loss is had, be it a breakup or a death in the family, the body wishes to cope. Our brain responds to this type of loss the same way it would handle a death, and even physical pain. Heartbreak actually physiologically causes physical pain. https://www.psychologytoday.com/blog/the-mindful-self-express/201104/the-neuroscience-relationship-breakups
It might sound clique, but only time will heal. Anything you do now should be a healthy distraction from your emotional pain. Spend time with your friends, your family, and focus on your schoolwork. You'll be surprised to find so many people so close around you that are going through the same type of emotional troubles. For some intense perspective, ask your parents about their teenage heartbreaks, talk to your professors about it (seriously you'll be surprised to find so much consolation from others)
Focus on your school work; at such a young age now is the time to really start to get your organization, studying, and leadership skills tuned up. Know that if you work hard to be something you really want, personal relationships will almost fall into place. Are you in college yet? Join more extracurricular activities or pick up a book written by a famous author from your Major / educational concentration. Work for the awards that are given each year for great students, volunteer at a community center, build your resume up!
Freinds and family: Dont be afraid to be honest. Don't tell people you're 'fine' - that word never flies ' use the word 'heartbroken' - open up to people and, again, you'll be surprised to find yourself surrounded by such caring people who are totally willing to give you their time of day if you only ask for me. Again, ask for it, dont be shy we all go through heartbreak, talk with others about their past relationships, constantly apologize for gnarring their ears off but best believe that they are happy to help you vent your issues. Venting is healthy. So vent.
If you're feeling down, watch a few of these, https://www.youtube.com/results?search_query=break+up+poem
If you're feeling up to it, write your own slam poem, or do what I do when I'm ruminating about my ex-gf, go onto the Relationship section of reddit and try to help others. From personal experience, having been in a handful of year+ long relationships, time heals, time clarifies, and time makes you wiser.
//On a more personal and direct note to your particular situation. Avoid talking to your ex-gf for now, there are still too many emotions tied to the thought of you two not being together and the thought of her with someone else. I'd advise you to refrain from contacting her in any way until a day in the future (few weeks, month) you two can talk civially, platonically, and maturly about the ending of you two. Right now its way to emotional there is no logic behind it besides feelings and thoughts. In time, try to have this constructive conversation to clear up any unknowns. Warning: these conversations usually never go as planned.
-Message me if you want to talk. Peace.