Is it possible for 已 to make a sentence into the future tense? According to my dictionary, 已 means already or/and afterwards.

However, for the translation of


  我已无法忘却。


, 


  I've already been unable to forget (it).`


seems to be a little awkward.

I seems this one is right one:


  I won't be able to forget (it) afterwards. (I cannot forget (it) any more.)


, but is it right to understand the sentence as the future tense?
已 means 'already' (not 'afterward', which is '之後' or '以後' in Chinese ) You don't have to worry about what tense it is in, because it is an adverb. 

Actually, you don't have to worry about what tense a verb is in, because Chinese do not use tense for verb, see this answer: How do you specify past tense for 是?


To use 已 in pass tense, you can write: "在三年前我已畢業" (I already graduated three years ago)
To use 已 in future tense, you can write: "明天走已太晚了" (leaving tomorrow will be too late already) 


Since the sentence "我已无法忘却" doesn't provide any time reference, you can read it as in present tense by default --  "I can't forget it already"

If you really want to  put "我已无法忘却" in different tense, you have to put time reference in the context that associates with this sentence.