Is the Quran really preserved or are there missing parts of the Quran?

First of all, I'm sorry for the nature of this topic but recently, my Imaan has become weaker. Shaytan is flooding my mind with doubts. This morning, I started to doubt the preservation of the Quran (May Allah forgive me). So I really need your help and guidence. I was reading about the history of the Quran when I noticed something interesting. During the Battle of Yamama, many of the huffaz were killed. So Abu Bakr told Zaid ibn Thabit to collect the Quran. And then Zaid said this: "*...By Allah, if he (Abu Bakr) had ordered me to shift one of the mountains it would not have been harder for me than what he had ordered me concerning the collection of the Quran... So I started locating the Quranic material and collecting it from parchments, scapula, leafstalks of date palms and from the memories of men*." Here's where one of the confusion lies. **If Zaid ibn Thabit was the personal scribe of Muhammad (s.a.w.) and if he had the entire Quran memorized, why did he have to collect Quranic material in written form? What's wrong with the material he already had? Did Abu Bakr not trust Zaid ibn Thabit's work?** Later, Zaid ibn Thabit said: "**So I started looking for the Holy quran and collected it from (what was written on) palm-leaf stalks, thin white stones, and also from men who knew it by heart, until I found the last verse of Surat at-Tauba (repentance) with Abi Khuzaima al-Ansari, and I did not find it with anybody other than him.**" This leads me to another question: **Why did Zaid ibn Thabit stop searching after he found the lasts verses of Surah Tauba? Did Zaid ibn Thabit find all parts of the Quran that was revealed to Muhammad (s.a.w.)? Were there any parts of the Quran that he couldn't find?** We all know that Zaid ibn Thabit's task was to collect written portions of the Quran and all the verses that he collected were unanimously agreed upon by the companions. **But what about the verses that he didn't collect? If Zaid ibn Thabit had some parts of the Quran memorized for which he couldn't find any written copies, were these parts recorded in Abu Bakr's compilation of the Quran or were they omitted?** Please reply soon.