Take a deep breath.
Keeping breathing.
And now reflect.
There is a reason that person is a part of your past and is not in your life anymore.Remind yourself of why. The only thing we can do is take all the good out of a bad situation and spin it to our benefit. You are who you are today because of what has occurred in your past. You may always love this person, you may always miss them. We just can not let this loss control us. There are stories out there of "high school sweethearts" and " 50th anniversary" kinds of couples. They are blessed. Many of us will have to go through a few people until we find the one that will last.
Just like endless jobs or perhaps unhappy living arrangements, boyfriends/girlfriends are sometimes only a small part of your life. I'm 24 and if I do live a long life then I have only experienced about 1/3 of my time here on Earth. This is my beginning. And even if its your middle or close to your "end", do not let something that was just a small piece of your journey hold you back.
Just hold on. The tide is bound to settle once again.