You say nothing happened between you and your ex. But something did happen to your current boyfriend when you left. From your boyfriend’s point of view, you broke his basic trust in you. Once that happens, it’s very difficult for the person who was left to recover. Ultimately, it is up to you to remain faithful, and it is up to him to decide if he is willing to forgive you and give you the gift of his trust again. For many people in his situation, that feels like a terrifying emotional risk.
You are both in your teens and just learning how to be in a relationship. That’s normal. The teen years and into the twenties are when people figure out who they are in a relationship and what kind of person they want to be with. It’s a very important developmental task.
You’ve already learned some hard lessons about what behaviors (on your own and the other’s part) makes for a good or bad relationship. Your boyfriend has the same developmental task to do. Difficult times can certainly be an opportunity for growth (both for each person and for the couple) but sometimes it is just too difficult. When that is the case, it is sometimes wiser to end the relationship and take time to think about lessons learned before venturing into another relationship.
You are both young to settle into a forever relationship. You may or may not be able to retrieve the easy trust you once had between you. If not, I hope you can wish each other the best and take what you’ve learned to the next person you want to be with.
I wish you well.