A lot of people unhappy in marriage, not just those who married early, are often-times unhappy from unrealistic expectations about marriage and/or their partners.
Young people also don't really know themselves, their own identity, apart from their own families.
In MY early 20s, I was dealing with an abusive family situation and I was a complete mess. I had no problem finding a partner, the problem was the mess and abuse was normalized to him. Why? Because we find partners that reflect what we're ALL used to.
I never got married. I knew it was a mess, things werent' right, and I focused on figuring out what was going on, then on fighting my abusers, then on repairing myself and figuring out who I am. That's taken up the rest of my adult life so far, and I'm about to be 40 this year.
It's not really a simple solution to a complex issue.
Like, I've heard in some talks, "Increase your faith in Christ and God."
Ok. So read your scriptures, make sure you're praying often, about everything in your life, check yourself that your heart and motivations are in the right place, that your energy isn't wasted.
"Be the partner you'd want to have."
"Work on yourself, your hobbies, your career, make friends in the meantime."
You know, all that stuff?
Other than "increase your faith," I found it to be shallow. It's a checklist without looking at the motivations.
Become someone who makes YOU happy. You make yourself happy? You will make someone else happy.
But if your whole motivation to do everything is to get married and make someone else happy so they'll make you happy? Inter-dependency? You doin it backwards.
Or, as Nephi says, work on the inner vessel before the outer vessel. And we can fool ourselves. We can say we're doing everything, working on ourselves to be a better marriage partner like we're making ourselves happy? We're not. 'cause the whole internal motivation is still backwards. It's still thinking that once that partner comes in, we'll be happy. No, the happiness comes from yourself, your faith, your journey with Christ first.
And when someone does see your worth, you will also be ready to see theirs, and grateful for what they're bringing instead of looking for the boxes to tick off the checklist.