It sounds like you both are comfortable with each other so I don't think that's the problem. It is hard to reach climax for a girl during intercourse especially since she has never had one that way before. But once she has her first one it will come more easily the next time. She just has to relax and allow herself, so anything you can do to make her feel more comfortable about it would help. Find out if she likes it rougher or more soft. When you give her oral or other stimulation and she does climax try to pay attention to how fast and hard or deep you are going and what she likes the best.
You can get to know her body probably even better than her. I say that only because my boyfriend opened my eyes and gave me my first orgasm and I've learned a lot about how my body works and what I like. It just takes time and communication. Sometimes different positions can help you last longer and might feel better for her.
In my personal opinion missionary is kinda boring. It can get the job done but not nearly as exciting and kinda difficult to climax that way. But everyone enjoys different things. Try having her on top and to switch it up have her reversed on top and laying down on top of you that way, I have found this very exciting and pleasurable.
I'm sure she will get there hope this helped a little! good luck!

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