There is a girl who studies with me for free. She keeps on asking for more and more. I feel guilty to deny her for the fact that I have a lucky life while she doesn't. But it's getting on my nerves now. Is there any way that I can deny her firmly ?

It depends on what she is asking about. If she is asking about more help on a certain problem, then you can deny it by saying that she should try to figure it out first on her own for a few days and only if she is really stuck should she go to you. Tell her that it is important for people to be able to figure out things on their own from time to time because you can't always be there for her in the future. Plus it's a well known fact that the things you teach yourself will stick with you longer than things that others teach you.

If she is asking about elaborating on certain subjects, you can recommend her a book. Tell her that this book knows more about this stuff than you do and that if she wishes to know so much, she should read this. Encourage her that one day, after reading everything in this book, she might even know more than you do (whether that is true or not shouldn't really be a concern). You could also recommend her to other people only if it you feel like it is necessary and there is no other choice left (I mean it's kinda rude to pass on burdens to other people but sometimes there is no other way around it).

I hope this gives you a slight idea on what steps you should take. I know that it's difficult to say no to people, but there are times when we have to think of ourselves too. I wish you luck!