I dunno...I kinda of see it as a friendship unrequited thing, tbh.
And I think I have been there...I was (I can't say dumped, cos it wasn't really a relationship), but a guy I wanted for a while didn't want what I wanted, but he promised we'd be friends & for a while we were, but then he made it really hard to contact him & finally - there was nothing. There was no "I don't want you, but you can't have anyone else" just disbelief (because of my naivete) that he wasn't all that into being my friend. And yes, his actions were low. Because he should have been honest & not lied.
So, with that done...
It isn't that he thinks she can't have anyone else, just that he hates the fact that after her promising to be friends (which in reality she doesn't want to do, so says it as lots of people do - to make the break easier for them both) she doesn't want that, but to be able to deal with going forward, he needs that. Even tho he said he felt lonely [in the relationship] & didn't want her love, he still wanted her friendship. And that she was low to promise him he would always have that!
I hope that make sense.
I can't see it in the sense that you see it at all, OP! I quite like the song.