Obviously you both shared a lot with each other over that amount of time. It is very common for couples to break up (Sometimes over trivial stuff) only to come back together again and realize they really need and love each other.
The real question is...Do you really love this young lady?? How much?? Are you willing to share more details about what caused this?? Is there another man/woman in the picture for either of you??
I am certainly not a relationship expert or counselor....not a great Psycologist. I was married to one person for 16 years before she passed away. I have not re-married yet. I can tell you that in my experience you will know if she's the "One" in a very short period of time because you will start to really miss her in even the most simple of ways. Feelings you thought you could simply forget, or bury with other activities or other people don't help. You feel empty and incomplete without her.
You feel this way and it doesn't lose any intensity no matter how hard you try to forget her....then I would suggest going after her again. If it was a mistake on your part, don't be afraid or to proud to swallow your pride and tell her you are sorry. Ask for her forgiveness and tell her how you are feeling. It is possible she may also be feeling the same way?
If you are positive you want to move on... be sure to do so first by counting your blessings, and remembering the great times you shared with her. Then go out and let time and life begin the healing process. Be open to love and it will find you again. Of that I have no doubt.