Are there any good fiction books with main characters that are 18-20?

There are a few good dystopian/sci-fi novels with protagonists more your age, so a lot less angst and whining. Try these.

Catch Me Falling by David J. Nix. As a caged display in a traveling freak show, 18 year old Crissa endures stares, taunts, and whispers of a terrible word from others: Human. But folks come by thousands to hear her do what no other can do. Sing! In a ‘what-if’ world populated by people who are not quite Human, Crissa's Human face marks her for abuse and grisly death. When a young man with a face like hers appears, she flees with him from those wishing to cage and kill her. Crissa’s flight carries her toward the mysteries of a murky past, her unusual face, and the wonderment of love and belonging.

These Broken Stars by Aimie Kaufman. Teenage Lilac is the only child of the richest man in the galaxy. Tarver is an 18-year old war hero, but a boy from a far inferior social class. When the two are the sole survivors of a spaceship crash on an unknown planet, they two must undertake a long journey to find rescue. Their initial disgust for one another slowly turns to love as they learn to respect one another’s strengths and weaknesses.

The Shore of Monsters by David J. Nix – 2011. Five generations earlier, a horde of monsters nearly obliterated humanity. All males are dead or ruined by a monster plague; words like 'father' and 'romance' have lost meaning. When teenager Sky joins an expedition to the shore that falls apart, she must survive amongst the monsters that roam the ruins. She gets unexpected help from a very surprising source. Mystery, action, and romance follow!

Blood Red Road (Dustlands) by Moira Young – 2012. In a post-apocalyptic future, 18-year-old Saba’s twin brother is stolen by black-clad riders. When tough-as-nails Saba launches a relentless search to recover him, she must fight for her life in gladiator cages, overcome enemies both creature and human, and learn to trust others for the first time. And try as she might, she can’t help but fall for the charming scoundrel Jack, who just may understand her more than she knows.

Incarceron by Catherine Fisher – 2007. Incarceron is a prison so vast that it is self-sustaining. Generations of prisoners are born inside, doomed to a life in prison. The novel weaves two story lines: that of 17-year old Finn, who hopes to become only the second person to break out of the prison, and Claudia, the warden's daughter who hopes to escape an arranged marriage. When each discovers a crystal key, they find that they can communicate through the devices, and pledge to help one another. The story lines merge in a fantastic twist you won’t see coming.

Noughts and Crosses by Malorie Blackman - In an alternate reality England, white-skinned people, called Noughts, are an underclass minority oppressed by a dark-skinned majority descended of Africans, called Crosses. Nought teenager Callum falls for Sephy, the daughter of a politically powerful Cross. As their romance grows, so does their safety in a society that does not tolerate racial mixing. When Callum's family is implicated in a terrorist act, the lovestruck teens face difficult choices. Regardless of your heritage, this story will lend you a new perspective on race and what it means to be human.

Shipbreaker by Paolo Bacigalupi – 2010. In a near future of rising seas, no oil, and extreme poverty, a teenager works as a shipbreaker - one who salvages rusting ships for parts. When he finds a wrecked super-yacht after a storm, he thinks his days of poverty are over. However, he gets swept into an adventure when bad people come for the one survivor of the wreck - a rich, beautiful girl who owns the vessel.

Have you tried reading The Giver? Maybe 1984 or Fahrenheit 451? Clockwork Orange even? These all have characters a bit older except for The Giver or A Clockwork Orange,but they are easily relatable. Try googling Young Adult SciFi novels. Do some research on it.

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I think the problem is, you haven't let go of the childhood need to read books about someone who is their own age or marginally older.

Young Adult books are not written for adults, they're written for teenagers and are called young adult, because this feeds their egos.