Can't help as I had the opposite experience but I eased into it. Went from clips and straps to clipless on my MTB. Picked up an old road bike to start on. Replaced it with a new, low-end road bike (which is now my commuter and still sporting the original saddle). Bought a new, Ultegra-equipped road bike and never looked back. Cycling clothing was a gradual move as well. And I started the process weighing in around 255 lbs.
Being comfortable in traffic is a gained skill as well. You need to realize that you belong there and have the same right to the road as the cagers. It is a bit intimidating to take the lane with a semi coming up behind you but it gets easier with experience.
That being said, it sounds like a fit issue as well as a saddle issue. I'd go back to the bike shop and have them help you get more comfortable.