You are not putting the paint on wet enough. You need practice before tackling painting the cab so here's my suggestion.
Get a quart of the same brand of paint in black because it's easiest to see orange peel with that color.. Get some 24" wide paper and tape a couple pieces overlapping to the wall and practice on that until you get a smooth finish. Intentionally slow down until you produce a run so you know how slow you have to go to make that happen. That will give you a benchmark for reference.
When you get enough confidence tape off your firewall down the middle of the line where the top weatherstrip goes and along the seam between the cowl and firewall and just paint the firewall using what you learned so far. You should see a marked improvement and since it's the firewall you can live with any minor imperfections.
You should apply one coat all over and then follow with another coat being sure to cover everything, then go back with another good coat to give you enough thickness to color sand, if necessary.
My suggestion would be to paint the doors, hood and fenders before the cab because the cab is definitely the most complicated part to paint and you need as much experience as possible before painting it.
Another thing is your gun. I've tried all the cheap guns out there and thought they were good until I bought a Devilbiss Starting Line set... HUGE difference! Get a good gun!
Fisheyes are a result of oil or silicone droplets on the surface. This is usually the result of painting in an area where you have used spray silicone or other oil such as WD40. Follow the advice previously given for cleaning the surface.
I hope this helps.
