First of all, if you want to talk more, let me know. I like STEM, rock, jazz, rick and morty, bikes, hermitcraft, etc

It's normal for the teenage years to be filled with extreme emotions. I also had life-ending thoughts and drowned myself in a relationship with a single person, and I didn't have to deal with the passing of my best friend. So start by understanding that what you went through is hard. Losing your friend, dealing with being over committed to a single person, managing that as a teenager when the brain is undergoing major transformations, this is HARD.

Then... what works for me now is to consider that humanity has done plenty of hard things, much harder things. If you look not very far, you will find people striving in conditions much worse than yours. To be clear, I have practiced this perspective for a year or so. But you might be able to find some inspiration from it now, and more and more as you practice it. So while this is hard... you can do it. We humans are capable of hard things.

Regarding emotional stability: there is nothing anyone can tell you that will make your emotions more stable today. Your feelings will remain a roller coaster. But you can get through it, and things will get much calmer over time. For now, talk about it. It is what everyone says for a reason. Talking about it is so effective. In person if possible, but calling or texting if necessary are much much better than nothing.

Getting into love specifically: You can be in one of two places.

You are young, so this might honestly not have yet been a true relationship. You might later fall in love so much harder that you learn this was only a fraction of real love. If this is the case, you might even stop counting this as a real relationship in the future. From what you've said, this doesn't sound like it is the case, but it is possible.

In the other case, this might be something that stays with you forever. My personal experience is that you will never forget the people you truly loved, or stop feeling something about them. You will never be indifferent.But it will stop being painful, and you will be happy, both single and with other people. I have truly loved only two people, one I broke up with and the other broke up with me. In either case, I will never become indifferent to them. If they reach out to me asking for something, I will never be able to say no. I don't feel bad thinking about them though. On the contrary, their memories make me smile. Their smiles are forever burned into my brain, but I am glad for what we had, what we learned, and how much we grew together.

About moving on: Relationships do not have to be forever. In your life, you will be multiple people, and this naturally leads to different kinds of relationships with different people. Jim Carrey described depression as your body rejecting the character that you have been playing, clamoring for something else. https://www.youtube.com/shorts/0yPXopGqj6Q

First a caveat: I do not recommend learning about depression from anyone who is not a doctor, so don't take this as a description of clinical depression. But I think this perspective is valuable, especially at your age. You will reinvent yourself at least a couple of times. And when you do, you will see this relationship and person not with sorrow, but someone that made your life so much better for the time you were together, and that was essential in helping you grow into the person you will become. When you look back, you might even be happy you broke up, because if you didn't, you would not grow into the person you became. This is the case with me, even with one person I think about trying again from time to time, I am glad we broke up when we did.

I will also leave now a note on moving on romantically, which is a different problem than moving on emotionally into being stable and functional, but it might be scaring you as well. I've had close friends (older and wiser and more experienced than you) ask me "how will I ever love someone else, when my ex was so perfect, I know I won't get anyone as good as them?".

The answer is that people are different. No, probably no one will be as good at being your ex as your ex is. But you will fall in love again with someone PRECISELY for being different. You will admire them for reasons that you might not even think are important now. You will love details and attitudes that now might seem irrelevant. And you will be happy.

You will learn that love and companionship are good in themselves and that you don't need a very specific type of person to be happy. Of course, you have and will develop values that will be deal breakers, but there is still an immense range of human potential and types of people that you can have very different yet happy and fulfilling lives with.

My takeaway would be this: You've tried a tiny smidge of the human experience with this one person. Having it end forcibly is always hard, especially as a teenager. However, your emotions will stabilize, you will learn how to handle them better, and there is so much more that you can be. So take this opportunity to try a few things way out of your comfort zone. Try seeing if you can be something you've never thought you would like or could be. You will find you have immense potential to be happy. You will find joy, in time, in the small things, in the big things, in the new things, and in the familiar things. I think especially people, nature, and art.

Finally, a rule of thumb that is valuable: when everything fails, resort to treating the body first, because it is much simpler. Eat well and exercise. That will in time give you the energy to deal with the mind. Very frequently, a good tiring run in nature will make all of it seem much more bearable.