I think it means that HE will eventually cheat on HER "I know its not mine, but I'll see if I can use it for a weekend or a one-night stand" os he has already "theres a heartache hanging from the corner of my girlfriends four-post bed" and he knows he's guilty and so does she but he's trying to lie to her "I know its not mine but I'll see if I can use it."
I think it means that HE will eventually cheat on HER "I know its not mine, but I'll see if I can use it for a weekend or a one-night stand" os he has already "theres a heartache hanging from the corner of my girlfriends four-post bed" and he knows he's guilty and so does she but he's trying to lie to her "I know its not mine but I'll see if I can use it."