That's why people pay for good work. **Coming from a guy how doesn't actually paint.** Try a little less hardner, run with medium pressure, and don't wait so long to get to the other side. You can jump back and forth by doing half and half, but you need to spray one side of the top within say 30 seconds to a minute(for that area), then hustle around to the other side and do the same. Nason will dry quickly, so put plenty on there and wet sand any imperfections out.
Don't worry if it doesn't come out perfect. No job will. Just slap enough clear on that you can sand out the imperfections. Besides, you shouldn't expect a professional job if this is your first real car your spraying. You got so many thing working against you like, no knowledge of materials, little knowledge of technique, little to no practice, spraying with no booth in a dusty shop, and tools that may not be the best. I bet your doing a pretty damn good job and just don't know it.
If you start to get frustrated with how its coming out then give it a rest. Let the Nason dry. Couple days in the sun, and then wet sand some of it down. Let the car dry, tack rag it, and try again. You're not painting in the ideal conditions with all the humidity that's in the air either. Unless your in a booth that dries the air for you, then you just have to expect some things you will have to work around. Put 4+ coats of clear on the car and you can sand out just about anything. GL