Just send her an e-mail and describe your feelings. It isn't personal, I know, but with an e-mail you can think about what to say and how, and when she reads it, she can think about it. I've done it and got a good relation for more then 2 years now. I know my girlfriend more then 6 years now, and it was just like you said, we started to do more things together, so I just told her my feelings and she felt the same