C182 pilot reports losing situational awareness approaching HEF from the southeast. Believing that he will be landing from a right base entry he does not understand the towers instructions to enter a left downwind for Runway 16R.

Narrative

Returning from a 3-hour trip to HEF; I was routed down the Dulles Airport eastern flyway with direct MULRR direct TICON. Virtually every approach to the airfield I have done in the last 9+ years has been from the south or southwest and I had only had this routing once before. This TICON routing saved time in that we didn't have to go via MRB; but it brought me to the field in a direction I was not used to. The last half hour of the trip was stressful in that I had to descend through a 3;500 FT thick cloud layer with turbulence; and then fly for over 50 miles at 4;000 then 3;000 FT in turbulence. My stress level was fairly high and although I heard the AWOS say the active runways at Manassas were 16L and 16R; I was expecting to have virtually a straight in from TICON as I had the only other time I got this routing. In my mind; I was landing from TICON to the airport and I was expecting a straight-in. I was so glad to reach the end of this stressful flight that it didn't register that I was thinking Runway 34; but the AWOS had said 16. When I was switched to the Tower frequency; I told the Tower I wanted the left side and expected him to tell me to enter a right base for 16L (seeing in my mind's eye the short runway heading north. Tower gave me absolutely proper instructions to enter a left downwind for 16R and that was fine; but I was confused and told Tower I didn't understand. They gave me additional directions and I tried to comply. They got frustrated; and I certainly understand why now. I just got turned around in my mental picture of what was happening. Fortunately there were no other aircraft at the field. I had a total brainfart; something which has never happened before in 20 years of flying; and which will certainly never happen again. My resolution to myself is to pretend I'm a student again and absolutely verify the landing runway; even on a perfect VMC day. Bottom line was that stress from the IMC descent and turbulence clouded my normally very good situational awareness. This will NEVER happen again!