Why am I feeling like this with my girlfriend?

We have been going out for a while, but not as long as you would think, so our relationship is still growing. But lately, I have been feeling a little ‘odd’.
For example, I keep telling myself, what if she finds someone better, what if she finds a guy that has everything she wants. Around 3 months into the relationship, I was speaking with her and asking her questions, she answered honestly saying that my height disappointed her as so did my age. But she fell in love with me either way, and that she got over it. She tells me I treat her like a real Princess, that she fell in Love with the right man. I tell her that I fell in Love with the right woman, that she is perfect in my eyes. I know she will never cheat on me, yet I keep feeling this way. I think, what if she leaves me because I’m not mature enough, or what if she finds a tall guy, or mature, maybe both. We have discussed this before, along with me telling her that I want to spend the rest of my life with her and her only. To have a family, and so on. We cannot see into the future, so obviously I am freaked out on that side also. I think, what if I lose her, I don’t want to lose her. I don’t know… I need help, is it jealousy? Is it that I’m just THAT afraid to lose her? I just love her so much, it scares me and it hurts me. I can’t lose this woman, I can’t.