她难过得说不下去了。 She's already said something, but she's too sad to continue. 她难过得说不出来了。 She's too sad to say it out, not even a word. 她难过得说不下来了。 We don't say that~~ We rarely put 说 here, before 不下来. But we have 听不下来， 撑不下来...which means the subject is incapable of finishing the action. In a word, generally speaking, V +不下去 can't continue doing sth. V+不下来 can't finish sth. because of your incapacity.

“她非常难过，说不下去了。” “他脑子太笨，说不下来。”

他说不下去： He can't finish his words (can be any reasons, such as, he is very sad about what has happened and can't help stopping the talking) 他说不下来： He is NOT ABLE to finish his words (maybe he is running out of ideas?)

她难过的说不下去了Or 她难过的说不出来。 “说不下来”is rarely used