Help! Trouble getting wet

Hi, so i have a problem that im a bit embarassed to admit. I cant seem to get wet during sex, or on my own for that matter. This isnt a new problem. I just dont get wet. Its not that im not enjoying the sex. I have no trouble having an orgasm. My girlfriend of course is aware of this and says it doesnt matter to her because she knows i still enjoy it, which is completely true, but i still feel embarassed like im letting her down. Ive tried looking up the cause but pretty much everyone wants to tell me its menopause. (Im only 18) if anyone is able to help me understand this id greatly appreciate it