I think the song is about suppressing the feelings of jealousy after a break-up, when both try to date new people and usually one of them (in this case the guy) still has the hope in retaking the relationship or at least there are remains of sexual attraction for her he cannot control
I imagine the next scenario. He goes somewhere with friends (a party, an event) and by coincidence he sees his ex-girlfriend with someone else. Someone tells him "they left together", he "runs out to go and get her" (impulsively he insists in finding out if she is indeed dating someone, even though they have ended the relationship) When he goes out, he sees her "holding hands with Trevor" (the guy she was with, he must be familiar to him, cause he knows his name). This affirms she is interested in someone else.
If you still feel attracted to your ex-girlfriend and you see her flirting with someone, it might be "not the greatest feeling ever". After that, maybe writing some other day, she suggests him to overcome the situation and let her go: "pull yourself together". She wants him to date someone else and to stop following her: "You should try your luck with Heather"
Then he heard "they slept together" and that breaks any hope to get her back. He finds out that "the less I know the better", because that makes him feel sexually frustrated and jealous (Maybe I deduce this from the ideas in the videoclip).
But he still has her in his mind and cannot forget her. He may still have feelings for her and suffers while imagining her with the new guy, "Oh my love, can't you see yourself by my side. No surprise when you're on his shoulder like every night. Oh my love, can't you see that you're on my mind", Maybe temporally he resignates: "Don't suppose we could convince your lover to change his mind
So goodbye".
She still gives him some small hopes, cause she doesn't want him to feel bad: "It's not now or never
Wait 10 years, we'll be together". And he agrees in waiting: "Better late than never. Just don't make me wait forever".
Then he points out how good he was doing after the break-up until that incident, when he saw her with Trevor by chance: "I was doing fine without you. 'Til I saw your face -now I can't erase- giving in to all his bullshit. 'Til I saw your eyes turn away from mine". He considers the guy like a dumb ass. He's in this state of anger and compares her to the person she used to be when they were together: "Is this what you want, is this who you are?"
He feels obsessed and try to question why Trevor wants her, if she only wants to use her:: "Oh, sweet darling, where he wants you". She considers, it's stupid to save the relationship like a superhero, saying silly things, when she has the possibility to move on, "Come on Superman, say your stupid line".
I think I could quite relate to the lyrics of the song, cause a week ago at a party I saw my ex-girlfriend with some other guy(we were together for 3 years). We had broken up months ago, but I felt intruded when I saw her flirting in the same place where friends and I were. I found out I still had some feelings for her (or at least I still feel attracted to her). After that incident I couldn't sleep one night, feeling jealous, wondering if the guy is better than me and so on. There was an impulsive that plays with your mind and makes you find out about her. I had the feeling that "the less I know the better", but some stubborn part of me was still insisting in convincing her of coming back.