A teenage girl I know commented on an image from The Notebook with a caption that went something like "Forever Love -- It is real." Her comment? "I would love for something like that to happen to me." It irked me. So I wrote a response.
A relationship that lasts a lifetime isn't something that just happens to you. You don't stumble into it. You work hard for it. Every day. You start working before you even meet someone, and you don't stop.
Learn to communicate. Not just talk, but actually getting across what you mean. Without blaming. Without accusation. Owning what is yours and accepting that's the only part you can change. Asking for what you need, even if you're afraid of the answer.
Be you. Really you. Know who you are. Know what you need, and what you can compromise on. Not every attractive person who likes you back is going to be a good match. Be willing to accept that. Also be willing to accept that not everyone is going to like you, and that's ok. If it's going to last, you can't hold back.
When you have a problem, solve it. You're going to disagree. You're going to have fights. You're going to hurt, cry, be angry, want to leave. Sometimes, they'll feel that way too. No one is perfect. Work through it anyway.