If you have not signed any representation agreement then for the most part you are in good shape. If you did end up purchasing a home that she showed you, or sent you information about then there could be an argument in her favor that indeed she procurred the home for you.
Be honest with her and thank her for the time she put in but that you have decided to go in another direction. You don’t need to tell her anything more then that although she may ask you about details as to what you are doing. If she asks to put you on an auto-search or spend any time whatsoever on your effort politely say thanks but right now you would rather not have her do that.
Just be up front and go your seperate ways. I have found that there are situations where I know I performed my best but have still had situations where clients feel more comfortable going in other directions. Recently I had a guy backdoor me and he ended up working directly with the seller of a home I showed him! Sh-t happens right? Anyways yep, cut your losses and go.
Did she show you this home that you want to make an offer on? How did you stumble on it? Did she have anything to do with your stumbling on it?