If I had sat with my back turned that way I wouldn't have seen it creep up on me!
If I had sat with my back turned towards the door I wouldn't have noticed the cockroach creep up on me!
"If I had 〜 (then) I would/wouldn't have 〜 " という文法を使って下さい。
past perfect （過去完了）+ conditional perfect (条件完了形）という組合となります。
〜 に背を向けて座る ＝ sit with (my/your/his/her) back turned towards 〜
忍び寄る ＝ creep up on