How can I stop these thoughts?

I (20M) and my gf (25 F) have been together for 3 years. I dropped out of high school and left my house at 16. I make ends meet as a fitness influencer. We go weeks without having sex due to our difference in schedules. My girlfriend is the only person I have ever had sex with and I feel disgusting for wondering what it would be like to have sex with other women. These thoughts started due to lustful comments on my videos from other women and messages inviting themselves to me. I do this thing where I make a profile on a dating app, like random profiles, and then delete the app. I don’t know to what avail I do this, I have never had one of these profiles for more than 5 minutes. Whenever I am just being nice to whoever woman, and she is clearly flirting, I make a comment about my girlfriend and how much I love her, and nothing escalates further. This has always been the case, as soon as I realize they might be mistaking my kindness for flirting I make it known I have a girlfriend I love. Currently, my biggest sexual fantasy since I am young can become true anytime I want (sex with a milf), she (42F) has told me she’d like me to come over her house to “really get to know each other”. We see each other at the library often, I do not have her number or socials (I lied to her and said I don’t have any social media) and nothing of this nature has ever been discussed. I have never mentioned my girlfriend to her but we would not really talk about our life besides books beforehand, I feel guilty for not mentioning I had a girlfriend, but I did not know she thought I was flirting (I wasn’t, I never am). I told her I have to go and she can have a nice day, but I keep wondering what that would be like as she summons incredible lust in me due to her shape and sex appeal, which I have never seen such in my short life. She is a model, that’s what she does for a living. This instance has occurred 5 times with women around my age or younger, and I have always made it clear I have a girlfriend and I am not interested. This is the first time I have not told the woman straight up I have a girlfriend, I left and I have not gone back to the library since, it’s been 1 week and I have not even masturbated because I know it would be thinking of her. I will not do anything about this but I don’t want to think about it. I feel disgusting, like I am a sexual deviant, like I don’t appreciate my girlfriend even though I do. I don’t understand this aspect of my mind. How can I stop these thoughts?

TLDR; I have a girlfriend but this other woman is on my mind a lot and I want her out. I keep wondering what it would be like to have sex with other women.