This looks like older, more archaic English. I would suggest that you read stories that were written more recently, because people don't speak like this. 1. "If it could but be managed" I think that this means "if only I could do it". " I would learn to do something if only I could manage to do it. The word "manage" can be used to mean "to be able to". "He managed to get accepted into one of the world's best universities", for example, means "he was able to get into one of the world's best universities". 2. "I should like to learn how to shudder" This means "I would like to learn how to shudder." He wants to learn how to shudder. Interesting that he would want to learn how to shudder; a shudder isn't a very pleasant thing at all. A shudder when your body shakes, and it is caused by horror, like seeing a ghost for example, could cause somebody to shudder, or tremble. It can also be caused by cold temperature or disgust. But of course, that's the current meaning of the word "shudder". Obviously, this story was written a long time ago, and "shudder" could have had a different meaning when the story was written. The meanings of words change over time.

I understand what you are saying. This writing form is sort of archaic. Basically just drop the but in your head and its the same If it could be managed if I could shudder adding but means that the if is sort of unlikely or difficult to do, but don't add but like that very often or ever. It is weird, especially for conversation.

shudder - trembling, shaking hmm still trying to think about the line "if it could but be managed".