This part is comparing a relationship to a drug. The good parts of a relationship (thinking about being with someone forever) are like the high, and when a relationship goes bad, it's like coming down from a drug. So, he's saying that everything was good, but it's going bad.
"I'm high on forever and always together"
This part is comparing a relationship to a drug. The good parts of a relationship (thinking about being with someone forever) are like the high, and when a relationship goes bad, it's like coming down from a drug. So, he's saying that everything was good, but it's going bad.