Using Weblio, I can go through the words and get some ideas on how to translate this sentence:
It: それは. I have spent enough time with Japanese sentences to know that I can just omit it and still be understood.
Was: でありました, であった，でした, だった, いた，あった, and a few others that I may have missed. Since this sentence is in writing, I’m trying to be polite, and the sentence has the basic sense of “X = Y”, I should probably pick でありました.
The most: 最も, 一番. I’m picking 最も, because it modifies an adjective.
Horrible: ぞっとするほどいやな, 実にひどい, 残酷な. I think “horrible” is being used in the “very bad” sense here, so I think 実にひどい would be a good choice.
Time: 時，時間, 時の経過，歳月. I’m picking 時間, because “time” in this sentence refers to a period of time.
Of: I know there are a lot of ways to translate “of” into Japanese, but I think の might be my best bet.
My: 私の. I think I can omit this and still be understood.
Life: 人生. That page is loaded with different terms that could be translated as “life”. Still, I have seen examples similar to the sentence I’m trying to translate, and they indicate that I should pick 人生.
Based on all this, I think “It was the most horrible time of my life” can be translated as 人生の最も実にひどい時間でありました.
Is this a good translation? If it isn’t, how should I translate it?