Isekai X Time Loop

God sends me to another world after I die

The king and the Princess begs me to kill the demon lord

I improved my magic and capabilities and finally slain the evil, I'm supposed to celebrate and relax after that, right?

So how am I back to square one, to the day I was summoned?

I did my search and tried again, It seems slaying the demon lord triggers a time loop, and somehow he has nothing to do with it.

what am I supposed to do? shall I live up to this loop or solve the mystery behind it?