I would definatly thin it out some more, a 50/50 mix usually does it for me. Though I haven't painted much since its gotton cold out.

make sure you sqeez as much paint out of the roller before you touch the car to.

as for the hardness, paint takes much longer to dry in the cold.

also you would probably benifit a lot from more sanding, I'd go from 120 to 400 to 800 before painting. Take it down until theres no more shiny areas at all, maybe even some primer showing through. Prep is always key.