Hello sweet Sheezy people <3

I have a question, how do you get used to make big drawings? Im trying to paint the mermaid challenge so I can one day print it on a tshirt and when browsing the net they give me these HUGE pixelsizes which are 3000px and bigger - and each time I try to draw that size I hit a wall. My whole being go ' This is uncomfy, image isnt acting like im used to and doesnt look like I want it to look... ' - and I dont know if its a me problem, a settings problem, or what.

I use Clipstudio when drawing, and a XP Pen Tablet.

EDIT - Update / Notes - I draw with 350 DPI since it was recommended, and the whole image is about 4200x3100 after following the printerpages guidelines.
First i sketch small, blow it up to size, and end up having issues with getting correct feel with brushstrokes and alike. When revisiting my old stuff I usually use 800DPI and draw like, 1000x1000 pixels.

Thanks in beforehand !