You asked how I know that there have been missing and added parts to the Bible. Well, consider. Until the advent of printing in Europe in the Middle Ages all copies of the Bible were done by hand. ALL of them. This resulted in both intentional and unintentional changes.
Intentional changes included:
(i)changes involving spelling and grammar;
(ii)harmonistic corruptions, for example the different versions of the Lord's Prayer in Matthew and Luke have been harmonised in some manuscripts;
(iii)addition of natural complements, for example including the 'scribes' with the 'chief priests' in Matthew 26:3 (by many copyists);
(iv)clearing up historical and geographical difficulties;
(v)conflation of readings (when a scribe found the same passage was given differently in two or more manuscripts);
(vi)alterations made because of doctrinal considerations;
(vii)addition of miscellaneous details;
Unintentional changes included
(i)errors arising from faulty eyesight;
(ii)errors arising from faulty hearing;
(iii)errors of the mind, which would include those that arose when the scribe forgot exactly what the older copy said between the time he looked at it and his writing it, usually a matter of seconds...but it happens;
(iv)errors of judgement, when words and notes in the margin of the older copy were incorporated into the text of the new manuscript;
If you want examples of any of these errors I would recommend another book by Bruce Metzger, namely The Text of the New Testament - Its Transmission, Corruption and Restoration (1964,OUP,pp.186-206) where they are well documented.
The result of these various changes, both intentional and unintentional, is that of the thousands of copies of the Greek NT no two are identical. Each one is different to a greater or lesser degree. If you cannot accept this then try writing out one of the gospels in its entirety and see if you can do it without error. Most unlikely. This does not mean that the text we have is unreliable. In most instances it is not difficult to establish the original text, as I tried to show in the case of the last chapter of Mark. But to suggest that the text has never been altered shows a complete ignorance of the history of the Bible.