How to get over heartbreak?

For context, I am pretty young. I had something with a girl I really liked for a while. She was my friend for a long time before we even got into something (I know usually not a good idea). It all started with going out a couple of times, and the that led to going out most days of the week. I found myself thinking about her a lot, and realized that I liked her. However something bothered me, she had just ended a relationship around 5 months before that. I knew she was not over it, even though she would always claim to be over it. I did not know what to do, do I try? Or do I choose to stay away. She clearly liked me too, or at least acted like she did. After getting advice from some friends, I decided to try it out. I told her how I felt, even though I knew this could mess up the relationship. She claimed to like me too and we had a great time for a while. Unfortunately my gut feeling was right. She eventually went back to her ex. One day she just turned distant, and never even told me that she went back to her ex. I was just the rebound guy who she even tried to keep on the side and stay as friends. I found out about this from someone else, and it hurt to know someone who I really liked, did not like me the same way I did. What I remember the most was when she told me, “i tried to like you, but I couldn’t” I felt like a loser for caring too much, and I evidently cut connections completely after finding out. Maybe it is not that deep, but for me it was. I think what hurt me the most was that it was just not some girl, but a close friend who besides my closest friends, knew the most about me. How do you let go of this? And how do I avoid this from shaping future relationships. I do not want to stay bitter forever, since it will only tie me to her. I know maybe this is not that deep, but as I said, to me it was.