Why can't I get the girl to say yes?

-Alright so I have been talking to this girl about 3 months now and i really like her. She likes me too bit everytime I ask her to go out with me she says no. We hang out together and act like boyfriend and girlfriend. The only thing I see that may matter is that I'm a year older than her. I really wanna know why bc she is amazing and I want her.why is she doing this? Is it bc she's scared?I'm not sure if this will help, but I was once in the same situation as the girl. I knew this guy really well and we both kinda liked each other but whenever he'd ask me out id say no on impulse. Im not sure why, it could have been kinda because I just couldn't see us that way, I was already with him a lot and a really good friend already. It kinda freaked me out and threw me off, I think.She can be scared yes, or maybe she's just busy every time you ask her out. Don't take it personally and try not to over think things.