I would say it doesn’t sound natural, because it isn’t possible to lock pain and grief, but it sounds poetic. Poetry doesn’t have to sound natural and there’s nothing wrong with playing with words. It’s figurative and I understand what you are trying to convey. I would just ask: Down where? Consider adding more detail to it. Other thing, does grief carry pain? Does pain carry grief? Consider keeping only grief, because there are multiple forms of pain… like muscle pain. But to grief is specific and I think there’s pain in grief. So, I would say, “Somehow, I could manage to lock all the grief down into the deepness of my heart.” I don’t want to put words in your month; just consider these corrections.

Yes but instead of I can some how lock… say I can some how lock down….

Hello how are you doing

I would correct it somewhat: I somehow managed to lock in all my pain and grief.

great way of describing how hard it was to lock down the pain and grief