As with anyone I might meet with, I would encourage you to ensure all is well on the biological level by meeting with your physician for a checkup. Have your testosterone levels checked, make sure everything is in working order. If you are relatively healthy, not on an antidepressant or anti-anxiety med, and can control for a satisfying relationship/partner, then you probably do just have a lower sexual motivation. AND that is okay. It doesn’t mean anything is wrong with you.
If your partner is looking for more frequent intimacy, perhaps you two can have a conversation around what that means to each of you and what that looks like. There could be a few ways to approach this matter, whether it is an uptick in frequency on occasion to accommodate both of your needs or enhancing the overall quality of the times you are sexual with each other. Maybe your girlfriend needs some reassurance that this is totally normal and not indicative that there is a problem in the relationship.
I challenge you to consider – have you always been less motivated by sex, or is this a newer situation for you? True, sexual interest can ebb and flow. It also drops off within a monogamous relationship naturally. So, could your relationship need an injection of novelty or inspiration to make things interesting again?
This can be a lot to process without some help. Consider finding a local professional to help you and your girlfriend navigate this if you find yourself getting stuck. Qualified professionals can be found through sstarnet.org or assect.org based on your geographic location. I wish you luck!