If it's also a difficult shift into first when the engine is hot and there are no noticeable problems with the other gears, then I fear we have a bent shift fork. The engine will have to come out, and the cases will have to be split.
But take it to a dealership and see what they say. I would REALLY love to be wrong about this, but judging from your typed description, a bent shift fork is what it sounds like to me. It probably happened when you were stomping it into gear on those cold mornings when the clutch wasn't entirely warmed up.
Pulling the engine and splitting the cases is a good deal of work, so it's not going to be cheap.
I personally would rather spend the money on tools and manual than simply pay some dealership to do it, but everyone's different. While you're replacing the shift fork, you can also get the gears undercut, which will make it the smoothest shifting machine you've ever ridden. I did this to my Honda Superhawk when I had it apart. My shift forks were fine, but I bought new ones anyway, just because I was already splitting the cases for a different problem. You'll gain a new appreciation for your bike after doing an operation like this. I learned that my Superhawk was an inferior design, but I still feel a little heart-pang whenever I see or hear a Superhawk go by.
Take it to a dealership and describe the problem without telling them about my shift-fork diagnosis. They might have some good news for you.
And if I was wrong, please come back and tell us. I would LOVE to be wrong about this. Good luck.
-Calamari Chris in Carlsbad, CA
http://www.chrisandlisachan.com/superhawk.htm
http://www.chrisandlisachan.com/hondasvietnam.htm