Penis gets hard only if I'm stroking it? ?

Are you watching porn? Are you addicted to porn? If yes to either question quit the porn. It has been scientifically documented that watching porn can affect male sexual functioning. If you don't  believe this statement do a little research. If need be seek professional help.

Either you aren't attracted to the girl you are with, or you are gay, or you have a medical issue.  See a urologist to discuss your problem. 

It's quite normal for the penis to become erect through physical stimulation. And there are some visual cues too. All normal.

If you cannot maintain an erection when you need it, there are other issues. I don't know enough to be able to answer fully in your case. You would need specialized consultation.

But it may be best to just relax, and let it happen as it does. Sometimes having an erection is not what is required at the time! Don't worry about it. (Easy for me to say)

You sound like you have an anxiety when it comes to sex. You have to just relax and let it flow naturally. Not really had this problem so can’t give you solutions but I would suggest trying some way to relax yourself before sex because if it’s with your partner she will understand. Just get her to give you a blowie that might help 🤣

whatever it takes .......................

 That goes by the lack of sexual experience, when you go to make, you become nervous, you full with anxiety, believe that you won't be able to, and that do not allow you to make what you want. You have to trust yourself, and have many desires to have sex, this way your penis will become as hard as a stone, it shot and it will continue equal of strong. He remembers everything it is in your mind, your penis ago what she indicates him, ahead that you can achieve it.      

ryan, i doubt that you have put it in  a vagina

take a multivitamin mineral supplement daily

you need to eat a lot of double patty

beef burgers

one tablespoon of honey daily

four or five pieces of Arabian Maryam dates daily

then only your cock will be hard like a rock when erect

This seems like a psychological anxiety about someone seeing you with an erection. Masturbation and vaginal sex are very normal, so your gf is fine with you keeping your penis hard by stroking it sometimes during sex with her. You can practice staying hard by just taking off your underwear, being totally nude, and masturbating to erection. Don't try to reach orgasm or ejaculate, but let it stay hard for several minutes, just touching or stroking it lightly. Practice daily, or frequently.

It should be able to get hard without you touching it. If you see a good looking woman there should be a rush of blood to your penis.