I would agree with this for the first part: Put White and hispanic for your race and ethnicity. Your looks don't define your cultural heritage or the environment in which you were raised.
I'm African American, but I was adopted by two white parents and I was raised in an almost exclusively white area (I was the only black kid in my class from pre-school through 5th grade with the exception of 1st grade.) Because of that I obviously strongly identify with "white" culture and even (wrongfully lol) said growing up that I was basically white anyway. However, I'm still an URM regardless of how I was raised. Since high school and coming to college I have have really discovered my black identity and have had the opportunity really explore what it means to be African American. I know that my situation is different because I obviously look black lol, but I think our situations are somewhat comparable.
Bottom line: My opinion is put white and hispanic. You do have some experience with Mexican culture and that alone is very valuable. I would put some time in and practice your Spanish because that is a skill that not only will be beneficial to your career, but also make your patients that much more comfortable with you. Also you seem to be struggling with this because you don't have strong connections with Mexican culture. You should take this as an opportunity to re-discover your Mexican/Hispanic identity. Reach out to your maternal family members, talk with your mother about Mexican culture, go to yours school Latino Culture Center if they have one. You're definitely Hispanic and you should embrace that! Sorry for the essay lol.