It looks like you don’t quite nail down a concept before jumping to markers, which aren’t as forgiving as a pen/pencil.
It’s easy to keep adding to one sketch, but sometimes its good to get those first ideas down then grab a new piece of paper and overlay w/ clean lines.
I’d say throw a bunch of quick sketches down refining a concept working on proportion (orthographic views help w/ layout/proportions) then jump back into perspective/markers… my two cents. keep it up tho and don’t forget to stay loose!!!