I was just the same when I first went knocking; I drove 5 miles to a village, drove around for a bit, and then came up with some lame excuse not to knock so went home. I felt bad for days.
So next time I forced myself to do it even though I really did not want to. You know what? I actually ended up enjoying it.
I think it’s the fear of being face to face and getting rejected, but I found most folk were really nice and after an hour I had 30 quid’s worth of work.
Once over that it’s a doddle.
Something that helped me was knowing that even if someone opens the door and is rude it’s not personal. I realized that after a guy knocked on my door selling fish, I said no but I still ended up talking to him for about 20 mins to pick his brains about door knocking, I even made him a cuppa.

He said 99% of people are fine but every now and then you catch someone at a bad moment and they take it out on you, "Like your neighbour"
I was amazed when he told me how rude she had been, she is one of the nicest people I know yet when he knocked she was really horrid and slammed the door. She had a 4 week old child at the time (her first) so I assumed he just picked a bad time. It made me realize that when you knock you have to expect it may happen but it’s not personal. So I am now able to just shrug it off and move to the next house.