You know your GF. Do you know what things to help her get “in the mood?” Maybe it is something as mundane as picking up the house or doing the dirty dishes in the sink without being asked. Something that allows her to relax sooner. Find out what her fantasies are and see about incorporating that into your foreplay.
Maybe she “gets hot” about being naughty in public. Take her to dinner. Someplace that has a dark booth. See if you can get her to dress in a low top that shows cleavage which you give her complementary praise - how much you think she is sexy. Maybe you work up to getting her to dress in shorter skirts and get her to take off her panties/knickers in the ladies room or in the dark corner booth you’re seated in.
This isn’t a punch list where you do these things and you’re guaranteed to get her off. It’s about finding out what helps her get in the mood.
Others may have ideas too. If you really like the idea of sharing, maybe you can find some homemade amateur wife sharing porn to watch together. Then find out if she likes the young guys the the thick cock, or the black guy with big muscles, or the red headed babe with the perky nipples. The key isn’t the porn. The key thing is for the two of you to COMMUNICATE.
Finally, here is the final thing. It,won’t always work day in and day out. Remember, it may take a lot more to get her horny some days that others. Communicate. And you need to let it be two way communication with you doing active listening.