I would say the absence of the relative pronoun "that" is acceptable in speech but maybe not in formal writing. The function of "that" would be assumed by most listeners
I've been gone for so long, that I almost forgot how beautiful you are.
I've been gone for so long, I almost forgot how beautiful you are.
The conjunction "and" brings the two ideas together but slightly opens the possibility that idea #2 is not due to idea #1.
I've been gone for so long and I almost forgot what roses smell like.
More interestingly is the question of mixing verb forms - present perfect "have been gone" with simple past "forgot". I think they should match, particularly as the "to be" (you are) is present. Therefore a fully correct form may be : -
I've been gone for so long, I have almost forgotten how to speak English.
I was gone for so long, I forgot how beautiful you were.
But finally in your example the implied context is "seeing you again and remembering", therefore it may be correct to use past perfect "had forgotten" because the forgetting is now in the past i.e. it has been replaced by "remembering".
I've been gone for so long, that I had almost forgotten how beautiful you are.
Man! English is hard
