I personally think it's about one of the lovers (the singer) on their death bed and singing to the other that he couldn't do anything he should've done before when it actually mattered, he only took it all for granted so he needs the other to remember him whenever he comes back in the form of a shrike to their sharp and glorious thorn and this metaphor probably speaks as a soul(shrike) that sort of depends on the other soul(thorn) because shrikes use thorns or barbed wires to stab their insects on it so that they get smaller bites to feed on. Also, the line "then when I met you my virtues uncounted" could mean that when he met the other, he sort of already started another life, more like a new chapter, where he forgets about all the past good deeds to make room for the new ones. And now that he's going, all of his good deeds stay with the other.
I personally think it's about one of the lovers (the singer) on their death bed and singing to the other that he couldn't do anything he should've done before when it actually mattered, he only took it all for granted so he needs the other to remember him whenever he comes back in the form of a shrike to their sharp and glorious thorn and this metaphor probably speaks as a soul(shrike) that sort of depends on the other soul(thorn) because shrikes use thorns or barbed wires to stab their insects on it so that they get smaller bites to feed on. Also, the line "then when I met you my virtues uncounted" could mean that when he met the other, he sort of already started another life, more like a new chapter, where he forgets about all the past good deeds to make room for the new ones. And now that he's going, all of his good deeds stay with the other.