Divine

I think that the biggest and most important part of this song is "loving you is the best thing and the worst thing in my life"

Obviously, if you truly love somebody, they will become the most important thing in your life. They will become that which makes you happy, and will make everything else seem meaningless, because all that matters when you love someone is being together. Hence, "Stay till the morning and that world might just get by". This means that with that persons love, everything else, even the world just seems to be, but not matter.

Now, as for it being the worst thing in your life. If you have ever missed someone, whether it just be you going away from your boyfriend or girlfriend for a week due to a trip, or even something as major as death, when you miss someone, it is usually for a good reason. It means you care for them, love them, but that pain from separation can sometimes be very stressful, and although the actual love that is shared is amazing, you can't help but be sad when you're not together, because you miss the happy feelings, hence those sad feelings becoming "the worst thing" in your life.