Well since everyone else covered the huge importance of how your situation is healthy, what you are experiencing is common. I was about to say normal but it isn't and I'd hope it's not normal, as something that depressing can be very detrimental to your health. You are not alone in this feeling, as I, too have experienced something like this. What you can do, like everyone here has mentioned is to move on.
Remember, your health and happiness is most important to you, and you are in control of what you do with your life. Not only would I take all the great suggestions from the above posters, but I want you to be more happier to yourself. This is clearly making you unhappy. Life is precious, and limited. Short is not really a correct term, compared to how long the universe has been "around" for it is but your life is something nobody else has an experience of except you and those you share it with. That said, let me give you an example of why it is important for you to be happy:
Life is finite, which means it is time-sensitive. If for example you were to live 100 years, but we'll convert 10 years into 1 bar like how they do animation frames, there will be ten frames. (10 years x 10 frames = 100 years)
So!
/???/???/???/???/???/???/???/???/???/???/
That is your life and what it could be, potentially. It can be either happy or sad. Let's say you are 20 and met this person and stay depressed thinking about them. It would look something like this:
Happy/Happy/Sad/Sad/Sad/Sad/Sad/Sad/Sad/Sad
That's not a happy life is it? There's a whole lot more to life than the above obviously but if you look at it in simple form you'd still be unhappy and miserable for a large portion of your life. I don't think you want that. We'll replace the Happy and Sad with Past, Now, and Future.
Now/Now/Past/Past/Past/Past/Past/Past/Past/Past
That's you living in the moment and being happy until 20, then living in the past because of that person hurting you. Take control and be happy, make your timeline look like this:
Past/Past/Now/Future/Future/Happy/Awesome/Sucksbutisanexperience/Livinglife/Happy.
You can do it. Trust yourself. Be happy.
