It takes a different type of maturity to decide who you will marry and to make a marriage work. You CAN’T say “Since I am old enough to get married I can decide who I will marry.”
Derech ha’olam is that young people get married and they stay married. This is the way of the world. Once you are married you gain a certain maturity through shana rishona, then through having children… and with middos tovos u BUILD your home. The ultimate maturity of marriage doesn’t happen in 2- 3 weeks like a shidduch might.
It is also derech Ha’olam that children listen to parents. It is a new thing that 18 yr olds are looking to marry themselves off. For THIS life decision, they may not yet be mature enough.
Thank your parents for their love and guidence. Don’t feel too haughty to accept their sage advice.
If you REALLY feel that your parents don’t have ur best in mind (hard to believe), Speak to a Rav or to a mentor for guidence but, girls/boys, don’t fool yourself into making such a major decision on ur own.