I want to express "patience" in the sense of "I don't have the patience lately to read a long novel." It's uncertain what exactly is the problem -- am I nervous? am I preoccupied with other thoughts? -- but the nuance of "patience" here will be understood by native English speakers. I've consulted dictionaries and read discussions about Japanese words for "patience"; but when I use them for this meaning of "patience", I get confused looks. What would be the best way to convey my intended meaning?
"I don't have the patience lately to read a long novel."
I'm not very proficient at English connotation, but based on the context below:
what I'm looking for is not really about "will power," in the usual sense of completing an unpleasant task, or refraining from an act of immediate pleasure. It's more of being in a state of mental equanimity that permits leisurely pursuits like reading a long book, or even putting together furniture from Ikea.
there might be several strategies to form Japanese expression.
[verb] + 気 roughly means "feeling like [verb]-ing" and やる気 means "will to do", so they can be used in the situation where you cannot bring yourself to do something.
They would sound like you become "short-winded" and cannot make it to the end if you try. 根気 may be closest to translate "patience" but it usually describes one's character that is not thought to fluctuate much, so 最近根気がない sounds a little off.
～ていられる has a nuance "able to keep an action (or non-action) for a longer while" that sometimes might be suitable for the word "patient". e.g. 待っていられない "can't wait (anymore)", "be impatient"
It is a freer translation where 集中力 literally means "ability of concentration" thus implies you cannot exclude other things from your mind and absorb yourself in the activity.
One word that came to mind was 余裕, which can be used to talk about "having room to" in a lot of senses - in your case, having the 'mental room' to do something that requires some higher level of thinking.
From a quick google search, it seems like 精神的余裕 is a pretty common phrase used to talk about things like mental fatigue.