I'm going to make my gf start paying some of the bills over a single incident
Will she actually pay some of the bills?
I heard she will never pay for it no matter how much I need her to do so
Instead of making the single incident your reason for her to pay, you should reason with her that it's only fair for her to contribute. She lives here too, so it's fair that she pays some of the bills. Using a past incident as justification is a weak argument and allows her to keep leeching.
I agree, I'm being too lenient anyways anymore. I'm only doing this for her own good.
Yes and remember never make it about the past. Just bring it up that these responsibilities need to be shared. That will help matters a lot in your relationship, hopefully.