Girlfriend cums when I stay still inside her. What else could I do?

As the title says, we start normal but when she wants to cum she holds me still deep inside her and grinds her clit against my pelvis, until she comes (takes about 15 seconds).

Now I'm happy for her, but I feel like I'm doing nothing, that the best pleasure for her is when I'm not trying to please her. She likes to do this in missionary, but I wonder if there's some other positions to try to do this, or of there's something I could do beyond holding still.

My understanding is that I hit a spot deep inside her, so moving removes that stimulation. Maybe there's something I can do to build up into the moment I press into her?

Any advice is welcome, thank you!

Update

Thank you everyone for the advice, I want to clarify two things:

For some reason too many people think that the issue is I don't feel involved, that's not it. I want to improve to learn to give her a better orgasm.

Also, since we are both inexperienced, asking here is a good way for me to learn new stuff. Of course I'm asking her too, but there's some things that can be learnt only through external sources.