This is a great question! (don't feel alone! Lots of introverts experience this conundrum, and ESPECIALLY in their early twenties). I know it's crazy, you turn 18 and you feel old, and then you can't believe it when you turn 21 and you think 'holy hell, I'm over the hill'. The truth is that you are perfectly on par with where you might as well be. Most relationships people have at your age don't last, not to downplay heartbreak: it definitely serves a purpose, but shit.. Who is rushing to experience that? Heartbreak feels like the lonely you feel now x by 5 big boat loads of ego crushing insecurity.
Another good thing to know would be that everyone reaches the optimum end game by being in an excellent relationship with themselves. There is nothing more attractive than somebody who is loving being single, and nothing more repellant than someone who can't handle it. Being single is a chance to prove yourself as a prize, and that is win/win, because if you do it right, you'll either end up with somebody you love to be with (a partner), or somebody you love to be with (yourself).
But in reality, you'll end up with lots of options and the point is, (especially for an introvert), is to choose another person who is strong on their own, so that you don't end up getting suffocated, which is your opposite potential. Stay centered, stay clear!
Good Luck!
“Love is the ultimate outlaw. It just won't adhere to any rules. The most any of us can do is to sign on as its accomplice. Instead of vowing to honor and obey, maybe we should swear to aid and abet. That would mean that security is out of the question. The words "make" and "stay" become inappropriate. My love for you has no strings attached. I love you for free.”
― Tom Robbins, Still Life with Woodpecker