Hey OP,
I somewhat agree with the others. But from the way I see it, this girl obviously cares about your friendship, otherwise she wouldn't agree to study with you or reply your texts.
No offense, but in the back of my mind, I am thinking that she is slightly on the clingy-side, and unable to make outright decisions.
For example, if this girl is considerate enough, she could have let you know in person that she values your friendship with her; and that she will require you to be more understanding since she has a BF already.
I've been in similar experiences before, and to be honest... I think she considers you the "nice" guy, and her BF the "bad" guy...but weird as it seems, girls likes the bad guys. Think of it this way, she received your texts and knows that you care for her, BUT her reaction is "meh, oh that's nice
"...
BUT, if you chose not to text her, she'll be thinking "hmmmm, how come OP doesn't contact me anymore
?"
She'll bounce back, so rest assured.
~my 2 cents