As far as I know, forms like 「てしょうがない」 and 「てしかた(が)ない」 means something like "very, extremely" and "can't be helped", like:
心配する理由はなかったけど、どうしても不安でしかたなかった: I didn't have any reason to worry, but I couldn't help be uneasy
Then I found this sentence:
which was translated as "You shouldn't read a book like that", while I translated it as "I can't help reading a book like that"; there is no context, it's an example sentence from a grammar.
I know those form can also mean "There is no point in doing something", but I'm not sure how to discriminate between those meanings. After reasearching a bit, I thought the difference was in the 「も」: 「てしかたがない」, "I can't help but"; 「てもしかたがない」, "I shouldn't". The again, I just found this sentence:
which to me sounds like "I couldn't help wasting time with such a drunkard", while it was translated as "I shouldn't have wasted time with such a drunkard", without having 「も」. I'm wondering if 「たって」 is a colloquial form for 「ても」: I know some time ago I found such a case, but I can't find it anymore.
I found some questions here about these forms, but nothing addressing this; in my grammar and on the Internet all I can find are the meaning I gave above, with no explanation about the "You shouldn't" meaning. I did find this page, in which there are examples like 「まだ証明されていないことを考えてみても仕方がないぜ」, translated as "Do not spend thoughts on a matter which has not been proved", which seems to prove my point, since it does have 「も」.
Am I missing something? Am I right in my
「てしかたがない」, "I can't help but"; 「てもしかたがない」, "I shouldn't" interpretation? Is it just depending on context?