. . . and I thought to myself, "It's easy to come up with healthy-relationship-alternate phrases for some of these":
Turn “I can’t live without you.” into something like "You make me happy to be alive" to remove the creepy "I'd die without you" implication.
What's creepy about “I know you better than you know yourself” is the direction it is being said in. If it's really true, the other person might say things like, "Sometimes I think you know me better than I know myself" or "I love the way you know how I like to be touched."
The last 2 are just plain wrong, though, they don't have a clever alternate.
“You should JUST KNOW…
…what’s wrong/exactly what your partner wants in bed/if someone wants to have sex with you/if you’re serious or casual or exclusive.”
No, don't assume you just know, Communicate, dammit!. Because it's so easy to THINK you know when you don't, really. Unless you communicate, and then you know for sure!
“If you really love someone, you want to keep them all to yourself.”
No. We all get possessive of things sometimes, but possessiveness doesn't mean you care about something, it just means you want it. There's so much more to love than that.