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.