When I create a new layer on top of the one I've already drawn on and put its mode to Overlay or Screen to change the color of my line work, I then merge it down with the line work layer so I can paint but the merging ruin the blend, instead only giving me not what I saw before the merge but my messy scribbling on the overlay layer on top of my line work

set both layers your want to merge to the same blending mode. Then merge.

You can group these two layers together and turn on alpha inheritance, which will make sure the overlay only applies to your linework. Then right-click and flatten layer. should look like this before you flatten it.

(you can also just go ahead and merge down, but flattening gets rid of the group layer as well.)

Thanks guys for replying and helping me with my issue

no problem!