English can indeed get a little confusing when you start looking at all the different time tenses you can imply in a sentence, depending on the grammar you employ. I will use your example (future tense) sentence and re-write it to show differing tenses:
"You could not make me happy, and I am convinced that I
am the last woman in the world who would make you so."
The use of 'could', 'would', or 'will be' all imply future tense. The past tense version would be:
"You could not have made me happy, and I am convinced that I
am the last woman in the world who could have made you so."
In this version, adding "have", "had" or "was" to 'could', or 'should' shifts the sentence into past tense. The present tense version of the sentence would be:
"You can not make me happy, and I am convinced that I
am the last woman in the world who can make you so."
"Can" may be used either as future or present tense, but using "is" or "am" almost always implies present tense.
One of the confusing things about these sentences is that her use of the words "I am convinced" actually sets that specific part of all of these example sentences into present tense; in other words she is convinced (in the present) that in the (past, present, or future, depending on the sentence version used) she (was, is, or will be) unhappy with him, and he with her.