Dear teenage bride,
So I know your conversation with someone else is none of my business. Still I couldn't help but hear you in the hall. I sympathize with the stress of your upcoming wedding, the timing of which is such that you need to plan it before graduation as there won't be enough time afterwards. And I know that it hurts that your mom has practically disowned you, buying your older sister a new car while telling you that if you are old enough to get married you are old enough to pay for your own car and upcoming college.
And yes, marriage is your solitary life goal, you don't know why you should wait to achieve that.
But as hard as it may be, you should step back and really examine the timing here.
I say this not just because he's a creepy old mormon dude and you're an eighteen year old girl. It's certainly none of my business what you marry. But you're eighteen, out of an general life expectancy of 75 years. You've lived some 24% of your life, all of that under someone else's roof. So less than a quarter of the way through, you are about to acheive the culmination of all your ambition, a complete fulfillment of everything you lived for. In graphic form:
The vertical lines are birth, marriage and death. The C years are your childhood living in your parents house and eating their food. The M years are your married years, living in your husband's house and eating his food. This is your life plan, and believe me, I've seen worse. All I'm asking is a very small adjustment, a mere 5% of your time.
The U is a quick and painless four years of University. At the end of that time you can marry your creepy old dude, or perhaps some younger college grad with real prospects and less of a propensity towards seducing high-schoolers. It makes little impact overall, like putting on your seatbelt before you hit the freeway, or grabbing a water bottle before aerobics class. Your future marriage will be richer and stronger for it, and your poor mother's frantic worrying will be set aside. Plus you'll have such an easier time arranging that marriage after college graduation rather than before high school.