Though the heart be still as loving, (3)
Even though his heart is still as loving, the speaker still doesn't want to rove anymore. It's like deciding to retire from baseball even though you're still at the top of your game. He's probably doing this because he knows "roving" leads nowhere.
And the moon be still as bright (4)
This is the first of two references to the brightness of the moon. Brightness, luster—these are things that get our attention, that attract us. Maybe the speaker loves the moon? Or maybe he associates it with love.
And love itself have rest (8)
Love can be tiring, and tiresome. Although love is a good thing, most of the time, sometimes it can be too much. Take five, Mr. Speaker.