I'm going to continue to sleep with a fwb despite her admitting that she has feelings for me even though I told her that I wasn't interested in a relationship.
Why are you going to get deeper into the problem?
The sex she gives me is good.
There's nothing wrong with two consenting adults having sex, but you're exploiting and leading this girl on. She has feelings for you, and you're using those feelings to get her to have sex with you. You should do the right thing and stop sleeping with her if you have no plans on starting a relationship.
I haven't told her about not wanting a relationship.
I think that you should be more upfront with her. Make sure she knows for sure that you do not want a relationship. It is not okay to lead her on if it is not going anywhere.