Yes, that is optional; but in writing, because of the interposed phrase from your teenage years, it would generally be clearer to include that. (You could do so in speech, but you could also indicate the structure of the sentence by your prosody).
And is not an alternative to that: if you use it, the structure is different, and you would need a resumptive pronoun: and you had completely forgotten it existed.
People do sometimes say such things without the pronoun, because we are making up our sentence structure as we go along, and sometimes come out with something inconsistent; but really it needs the it.
Further, I would say that (in all its forms) it needs you before "actually liked".