Oof, paying for OF while in a relationship? That is pretty bad honestly. I swear man, women can just smell if you have been looking at porn or not. Still, it aint over until the fat lady sings. I would attempt to work it out with her if you truly love her, if not C’est la vie.
I have been in a similar situation before, not paying for OF but just this fetish in general, my two exes were both ex-fat girls who lost the weight so they really did not like it, luckily the girl I am with now was anorexic and wants to gain. Still I have had these conversations before. Merely talking about it wont help, women want to see actions. Show true repentance by acknowledging you have a problem and start actually demonstrating it (not just talking about it) by making some changes. This could be canceling the CC you used for those purchases and deleting stuff locally saved, less time on the computer, etc. When discussing it with here I would not recommend you “barter” with the fact that you will change, but openly share you are trying to drop the habit BUT I would also not deny what you are attracted to. To do otherwise would be dishonest and she will know. Women view any looking at other women as cheating and dishonest as well, but you still shouldnt lie and say this stuff isnt attractive to you if that comes up. Your problem IS your porn addiction, NOT being a chubby chaser, keep that in mind. Dont be dishonest with yourself or her. Basically just dont lie, she will find out and it makes you look weak. I would also give her some space and allow the dust to settle a bit, when she comes to talk to you about it (and she will come to talk to you about it, women love group consensus) you can make your ultimatum. She will have to make peace with what you are attracted to or you both part ways.
In the future, whether you stay with her or find someone new, make sure you keep her happy, preoccupied with what she likes to do and well fed. The pounds will naturally accumulate, keep her confidence high enough and she will eventually make peace with being thick/chubby/fat and enjoy being attractive to you. Turn your girl into someone you cant keep your hands off of and you wont even feel the need to look at porn.
Just my $0.02 fwiw, hope it helps.