I know that some people would have a problem with my answer... but I have to tell you that my first love, I met when I was 17.  The guy Adam was 26.  We were actually just friends until I was 18 because it just didn't seem right to be anything more than that.  But we were happy for 3 years and he's still a really important part of my life.  Yes, being 15 and 18 is hard.  But, maybe for now, you could just spend time together and make sure that you don't get carried away.  I wouldn't say it's a good idea to get into a super sexual relationship because of the fact that she's not of age.  But you can be together.  What does she think?