How can I train myself to stop falling for girls?

A 20 year old guy? Good luck with that! We're hard-wired to be attracted to women. For the next 50 years anyway, and counting. A better approach, imo, would be to figure out, perhaps with the help of a therapist, why you would want to deny yourself one of the greatest pleasures in life.

Go to the animal shelter and get a cat. Then every day train the cat to not eat a tasty piece of meat. Just put the meat down and when the cat tries to eat it, smack it. It'll take a long time, but eventually the cat will learn.

Once the cat has learned, then leave the piece of meat and leave the room. And see if the cat still wants the meat.

You really can't fight basic biology.

Well I think that you can t exactly stop falling for girls but I do think there are ways to stop falling for them.For example,focus on the bad things about a specific girl.But in all honeslty,if u fall for a girl you find that something about her u like and I cant even imagine why u wouldnt want to fall in love.Maybe instead of trying to find a way to stop falling for girls you can find out why u dont want to fall for her and move past it so a relationship can starts and maybe lead to a happy ending.

Get married for a while, then divorce. That should fix you.