size!? What can i do? How much bigger?

Hey, my name is Mike and i am 15 y/o. Here's the thing. Me and my girlfriend Jess have been going out for about... 6 months now and a couple weeks ago we decided to start having . The first time we had was great (on my end) but i could tell Jess wasn't moaning from the pleasure but from the pain. We talked about it but she told me that it hurts for most girls when they have for the first time. We've had 12 times now and the pain doesn't seem to stop. We talked it over and came to the conclusion that my is too big. I knew i was going to inherit this because, unfortunately, i have heard rumors about my older brother. We measured my and it is around 7 inched give or take. How can we fix this problem? Is there any way that we can make this enjoyable for BOTH of us? And also... how much more will it grow? I am almost 6 feet tall and again i am 15 and my is about 7 inches long. Please help if possible. Thank you!!