She needs to give you a blowjob, while pumping the blood upwards by making a circle with her thumb and index fingers, and suck with her mouth until you get hard. While she does that you need to squeeze your dick muscles. You can teach her how to do it. Another way is just to squeeze on the tip of the dick with the fingers, the lack of oxygen in there will provoke an erection.
Yes, the bottom line is you can't both be sitting there just looking at it waiting for it to get up, that's not how it works. She needs to be more affectionate or you need another girlfriend, more mature.