I had your same problem and I could solve it, it is highly illegal but it's definitely worth it.
Long story short, nicotine. It is highly addictive and with a little bit of Pavlovian manipulation you'll make your cold fish checking-the-phone-while-fucking gf into a slut to whom your semen is a staple part of her diet. Go to your nearest vaping store and buy odorless vape liquid. This normally comes into a concentration of 2% nicotine. Buy 5 and in an unused pot on low temperature, simmer those 5 bottles into the volume of one. Never use this pot to cook food ever again. Now you have vape liquid with a concentration of 10 % nicotine. This is so the concentration can no longer be replaced by normal tobacco. Btw if your gf doesn't smoke, start with a lower concentration. DO NOT GO OVERBOARD WITH THIS STEP. Large doses of nicotine are lethal so don't ever try to raise the concentration further or you could actually kill your gf.
Put this into a small, discrete spray, and stealthily put a little on your penis ONLY prior to penis in vagina sex. Nicotine is absorbed through the skin, and soon your gf's brain will associate the nicotine high with sex. Yes, you'll get addicted too, but you know you're addicted to the nicotine not to the sex, and frankly is either that or fapping in the bathroom at the dead of night. After a couple of weeks, when your gf will be hooked you'll notice a great increase in her desire to have sex. When she starts developing tolerance to nicotine, you'll basically have a common gutter whore at your beck and call. Bonus point: Since your slut gf is addicted to nicotine and not actually sex, when (not if) she tries to cheat it'll do nothing to her and she'll beg you on her knees to have her back. I don't need to tell you this is the point in time when you no longer need to be reasonable in your demands. When my gf pulled this off I forbid her from eating anything but my cum for 2 days and took her anal virginity in facialabuse.com style.
