Even if you may not feel it, if you have had more than one love in your life, consider yourself lucky. While you may not realize it, they taught you everything, which you seem to realize when you experience your second love. But at the time your second love comes along, you know what love actually is, how to handle it, how to express it.
You know what to expect. You know that everyone may unfortunately not have the same heart as you, but you are also optimistic enough to know that everyone is different and everyone has the ability to show the same love that you give them. Your first love taught you that and hopefully taught you how to express your feelings.
In my case, my first love taught me to not give a damn about what other people thought, that if I was happy, we were happy, and we were pushing each other towards greatness, that's all that mattered. Realizing that this person taught me everything is eye opening, which is exactly what a first love should be about, at least that's how I see a first love. This person pushed you, made you believe in fairy tales, showed you how a single person can care for you so much, even if for such a temporary time.
While your second love or ones after that may last much longer than your first, or possibly forever, they will never compare to your first. They won't be the ones to first teach you everything you need to know and everything you cannot possibly learn in a 900 square foot classroom of high school. They may teach you that not everybody leaves, but sadly, they weren't the ones to impact you and change you at first.
Although the way things might have ended with your first love still make you mad, angry, or hurt, and they still may not deserve happiness, be grateful for them because I know I am more than grateful for mine.