Fact: Most people die alone... in hospitals, no less. Sometimes they were widowed, some never married, sometimes the spouse is just at home.
You're 18, not 81. You have your whole life ahead of you. You have no idea if you'll suddenly stumble onto some great relationship at, say, 25.
Alone or not, there's plenty to do, explore, enjoy, learn, and accomplish. Romantic relationships make up only a small part of life. And to the person who doesn't see the good in life past 35? Horse s**t. The teens and twenties are said to be the most challenging parts of life, where you develop and get to know yourself. The thirties and forties are where you have wisdom, maturity (not "omg, age 35 and it's all over"), confidence, and the ability to take life for what it is, laugh, and enjoy yourself.
Learning to live alone and to enjoy your own company is the best advice I've read in this thread.