All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The treatment for this fear is to think good of Allah Almighty and to know that He is Most Merciful to His servants, Most Gracious, and Most generous.
You should strive in supplicating Him earnestly and sincerity to guide you to the Straight Path.
You should know that the true belief is absolutely the one upon which the Prophet and his Companions were.
We do not believe that we are infallible; we are humans who make mistakes (commit sins) and do righteous deeds.
However, the principles of ‘Aqeedah (faith, belief) were thoroughly and meticulously illustrated by the Prophet in a manner that did not leave any room for any criticisms or condemnations.
Whoever looks for them will find them in the Quran and the Sunnah. Besides, the pieces of evidence on its authenticity (and truthfulness) are clear and straightforward for whoever ponders on them.
So, you should do your best in learning this ‘Aqeedah while having a firm belief that it is the only way for salvation.
The characteristics of Ahlus-Sunnah wal-Jamaa’ah is that they do not provide evidence for their beliefs except with the Quran and the authentic Hadeeth, whereas others provide evidence of their whims and desires, or with Islamic theology, as is the case of the deviant sects. So you should act according to the Book of Allah and Sunnah of the Prophet and take this as your methodology, and you will never be misguided if you seek guidance from them.
Therefore, seek the truth with sincerity while thinking good of Allah. If you do so, you will never be disappointed.
We ask Allah to make us hold fast to His Book and the Sunnah of His Prophet until we meet Him.
As regards your claim that other groups or sects are more numerous and greater in number than the Ahlus-Sunnah, then this is not true.
The creed of the majority of Muslims is mostly sound and in concordance with the Fitrah (the natural predisposition upon which Allah created mankind). If we presume that the matter is as you mentioned, then a small number or a greater number is not the correct balance for the truth.
The real balance is to be in concordance with the Book of Allah and the Sunnah.
As long as you have learned the evidence of that creed from the Sunnah, and you have known that it is completely justifiable, as you mentioned, then repel the whispers of the devil and the doubts which he casts in your heart.
You should be reassured, have peace of mind, and think good of Allah.
You should know that Allah has forgiven this nation the sins that they do by mistake. If you followed a trustworthy scholar in a view that is not correct, then you are excused in the sight of Allah.
May Allah guide us to do whatever He likes and is pleased with.
For more benefit, please refer to Fataawa 33503, 2496, 22613, 40747, 351652.
Allah knows best.