no must be I felt really guilty until I explained the situation to her past perfect is used to express "action before action in the past" but you have two consequent actions: felt guilty => explained

I think the most appropriate option here is "Until I explained her the situation I felt guilty. From the grammar perspective in such sentences you could use past perfect to describe the action which occurred first. For instance, I had finished my meal when she arrived (at first I finished the meal and then she arrived) But in your example at first you felt guilty, and afterwards you explained her the situation. Moreover since you use "until" in the sentence it is oblivious for a reader which action occurred earlier, so you could just use past simple for both verbs.

Original sentence is fine. Although I would probably put it the other way around: I felt guilty until I had explained the situation to her.

I don't know about the official names of tenses, but your sentence is fine as it is.