Rules for “V’sein tal u’matar l’vracha”:
1. If you already said Hashem’s Name at the end of “Barech Aleinu”, you don’t go back. You say it in Shema Koleinu instead (Dirshu Siman 117, f.n.#20)
2.If you forget in Shema Koleinu, as long as you didn’t say the word “Retzei” yet, you go back to Shema Koleinu (SA 117/5)
3. If you already said the word “Retzei”, you go back to the beginning of “Bareich Aleinu” (ibid).
4. If you already said the Yihi ratzon at the end of Elokei Netzor, you go back to the beginning of Shemona Esrei (SA & MB 18).