Presumably he really likes this other girl, and it's a really physical desire ("I want you like I'm eighteen" and "In another life I'd be drenched in sweat with you"), but he's already with someone else ("I'm tied to my baby"). So they have to "get over it" as in their desires, because "it's this life darlin', and in this life we make do" - you can't always have what you want in this life.
Presumably he really likes this other girl, and it's a really physical desire ("I want you like I'm eighteen" and "In another life I'd be drenched in sweat with you"), but he's already with someone else ("I'm tied to my baby"). So they have to "get over it" as in their desires, because "it's this life darlin', and in this life we make do" - you can't always have what you want in this life.
Really like this song - very catchy like you say!