You may not be her first, not the last and not the only one. She loved before she fell in love again. But if she loves you now, what else is wrong?
It is not perfect, but you too, and you will never be perfect together. But if she makes you laugh, the more you think twice; if she gives you the opportunity to be a man, make mistakes, hold on to her and give her everything that you can. She can not think about you every second of the day, but she can give you a part of herself, because she knows - you can break her heart.
So do not hurt her, do not change her, do not analyze and do not expect from her what is higher than her capabilities. Smile when she makes you happy, let her know when she's angry with you, and miss her when she's not around.