I had been sober, but I drank for the first time in a while.
ご質問ありがとうございます。
「酒を断っていた」は辞書的にそのまま訳すとI was refusing alcoholとかI was not allowing alcoholになりますが、この二つの言い方が不自然です。それらより、I wasn't drinkingとI was soberの方がいいです。
他の英訳：I wasn't drinking alcohol, but I had a drink for the first time in a while.
ご参考いただければ幸いです。
I had a drink last night for the first time since I stopped in February.
この場合、次のような言い方ができますよ。
ーI had a drink last night for the first time since I stopped in February.
「2月にやめて以来、久しぶりにお酒を飲んだ」
I had a drink last night for the first time ... で「久しぶりにお酒を飲んだ」と言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
