Need some more outside opinions..?

So, I have already asked some questions about this girl I am involved with. But just to fill the people in who are reading this, She is everything. I have known her since she was in seventh and i was in eighth grade. We quickly became best friends and now i am a few weeks out of school from graduating, and she is going into her senior year. It was about ten months ago when i decided to talk to her about some feelings that have been growing inside for her. I was completely unsure about them at first, due to the fact that she was my best friend, and still is of course. But i was worried about that at the time. I brought up the courage to talk to her and it seems as if ever since the first moment we met, there has always been some sort of connection.... I dont know why i didn't realize it at first, but for some reason we have always had a strong connection. This is all, of course, described in a condescend version... If i could i would say everything about us, but it would just take too much space. But ever since then things have grown in a completely unremarkable way.... I could have never imagined having something like this happen to me, or this early in my life. It caught me off guard, and now i am utterly astounded about how much we love each-other. For the first time in my life i love somebody. And i mean really love someone. I know for a fact that she is in love with me. I can tell with everything she does, as well as says. We have come to a point that we are both dead set on getting married later. We both agreed on it, she is actually the one who asked me about it first xD. Well anyways, the only predicament i am in, which isn't really a big one... But it has been on my mind for a while. I am going on a mission here in like, five or six months, and i am thinking about proposing before my mission..... We have talked about this several times, and she actually likes the idea. The only issue i guess is i dont want her to feel trapped. She always tells me, i would never feel trapped with you. The only thing is, i can almost guarantee that at some point a guy will come along and he might have some sort of connection with her. I am not doubting us at all, but all i am saying is, i want her to feel like she can do whatever she wants, without feeling guilty. If she ended up kissing another guy or something while i'm gone, and she is engaged to me she will feel horrible. I also dont want her to have to be engaged to a guy for two years and have no contact with him, only through letters and the once-in-a-blue-moon phone calls... I would really like to do this.... For those of you that think i am going way to fast, i can understand... But being best friends with someone for almost 5 years and falling in love i feel is plenty of time. Again, i am not going to do anything forceful. If she decided to go with another guy, then she does.... I can't stop her. But that doesn't mean i will give up without a fight..... She is well worth everything in this world.... Actually everything in this world doesnt even compare to the worth she is to me. Since we both are very into the LDS church, we have prayed so many times about our future, and it feels as if we will be together. Those of you who dont belong to the LDS church, sorry that you have to read this, just deal with it xD. Anyways, I do love her so much. She is my very best friend, and my love. If i could get some help, that would be fantastic.