You may not be her first, her last, or her only. She loved before she may love again.
But if she loves you now, what else matters? She's not perfect - you aren't either, and the two of you may never be perfect together but if she can make you laugh, cause you to think twice, and admit to being human and making mistakes, hold onto her and give her the most you can.
She may not be thinking about you every second of the day, but she will give you a part of her that she knows you can break her heart.
So don't hurt her, don't change her, don't analyze and don't expect more than she can give. Smile when she makes you happy, let her know when she makes you mad, and miss her when she's not there.