At your age , you not being able to get hard might just be " performance anxiety " , just the pressure . Spend alot of time in foreplay with KY or a good silicone based lube . Take your time , enjoy each other . Penetrate with 1 ,2 then more fingers and stretch slowly , not to cause any discomfort .
Definitely invest in some KY Jelly for both you and her. It definitely helps with the insert part. But remember to reapply later on, especially being that its her first time.
Yeah. the same thing happened to me. We had been together for four years, but for three months me and my boyfriend (both 21) both virgins, couldn't figure it out how to have sex. I thought I had a vaginal condition called vaginismus, in which the vaginal muscles are so tight that the pubic bone makes it impossible for the penis enter, we used lube and everything... until I realized that...we were in the wrong spot lmao *amateurs*... lol. I laughed at that in the end. However, that may be the problem. Or foreplay, the vagina has to expand several times internally before it can accept any penis, most men don't allow enough time for foreplay which is essential for most women and it does take time. But it would help to see a doctor. It may be vaginismus in which they would recommend vaginal dilators and Kegel Pelvic floor exercises.
Take it slow. If oral satisfies both of you then it is a great way to start. Once you are ready to move on, I would use a lubricant. Try not to become so focused on penetration, it will come when you are both ready. We can play mental games and this can cause lack of performance.
Here's some obvious advice - have you tried lubricant? Masturbating while you're with her (to maintain an erection)? Try lubricating your fingers and manually arousing her, to help her relax, self-lubricate, and loosen up. Then try penetration.