Honestly, don't overthink it and psych yourself out of it, just say it. Find a suitable time, like, hang out with her one on one, and when the time is right, just tell her how you feel, especially if you're quite sure she likes you back. (Make sure you actually tell her on the day though!)
About the L word:
If you're not that close friends with her, then imho, avoid the L word at all costs! if you tell someone that you love them when you don't know them well (or at least, others wouldn't consider you guys close friends), it comes off as infatuation more than real love, which is obviously Not Good when you want said person to reciprocate. If you're read/watched Pride and Prejudice, think of how Lizzy reacts to Darcy's "profession". It was weird, a little creepy, and hilarious. Not. Good.
If you're shy about saying it, or you're unsure if they love you back, just tell them that you "really like" them, with feeling of course. Meaning is obvious, but not as overwhelming as "love". Alternatively, TRex's suggestion
is a really cute notion, shows that you like her so much that you're too nervous to actually tell her, which comes off as really sincere. But then again, this depends on the girl...it could seem like you chickened out...
Oh! And I don't know what others think of this, but I think if you also tell them
why you like/love them, that probably comes off more sincere too! Something like, "(Name), I think you're (amazing, a beautiful person, etc), and whenever I'm with you I feel (something original and sincere). I just wanted to tell you that I (really like you/think I'm in love with you)." Then maybe ask her how they feel.
In the OP's case, it seems as if you're quite sure she likes you back at least. So don't be afraid to tell her, otherwise you might regret letting the chance slip by. Good luck!!
My two cents.
