that when riding fixed, hills, while they are difficult sometimes, don't present the kind of obsacles you would imagine? Only having one gear and all.
I think you are right, in that there is a big psychological element. Roughly, if you absolutely can't gear down, you have to just get on with it. when I was a lot younger, I rode for one winter on fixed. I had a Carlton Corsair with a double sided hub, so I flipped it and rode it fixed, just to see if I liked it. I was riding 16 miles each way over the Yorkshire Wolds, so plenty of hills but nothing massive. I think I opted for around a 72" gear. I was honking like mad up the hills and windmilling down the other side, but I never had to get off and push, and my strength and cadence improved massively. I am 30 years older now and probably a bit less fit, but my current bike goes down to 27" and I use all the gears on similar terrain. I imagine that if I only had one gear I would probably manage - must try it again one day. Psychologically, when you have to do it, you do it. I think your point about riding at night is correct - in the dark, hills have far less mental impact as you can't see them.