Love
Kurt was sure what they had was love. He's been in love many times (mainly with straight men, but still) and he's never felt like he did with him.
With Blaine.
But if Love is really what they had, why did he do that? How could he just cheat on Kurt? It went against everything they had promised, not to mention it was grossly out of character.
But despite his ever growing relationship with Adam and the fact he should not still love Blaine, he still did love him, more than anything.
Kurt loved Blaine, that was clear as glass to anyone that bothered to look into it, and Kurt would openly admit it.
He couldn't forgive him.
Just Love him.