PhD manuscript with grammatical errors and informal phrases I have made a very good contribution in my PhD and just graduated two years ago from a top university with honors.

Today, I revised my PhD manuscript and surprisingly found several grammatical errors, informal sentences that I don't feel comfortable with now, and honestly also found two technical errors (they are correct in my published papers but not in my PhD manuscript). I am really disappointed, and feel like "I don't deserve to be graduated from a top university" with a PhD manuscript that contains a lot of grammatical errors and several informal things.

When I wrote the manuscript, I was happy with the informal things I added, but today I feel really sad by having them in my manuscript since I think they decrease the reputation of my work. 

Although that after my defense the committee gave me one month for the minor corrections, but I don't think I have really benefited well from this opportunity. Is this normal? Any suggestions?
Ok. Let the past be the past. You weren't perfect then, you are better now. Maybe not perfect yet, but neither am I, or anyone. The very fact that you can recognize past errors is a sure sign that you are now better at this than you were as a novice scholar. 

As the comments note, there is little chance the problems will be noticed, or even that they will matter if the are noticed. 

There is a phenomenon among writers that is worth noting. Many people, writers of all kinds, sometimes feel embarrassed by their older work. Even poets. It is a sign of growth, not of failure. 

In Tai Chi we say "One day's practice, one day's progress." 



A couple of additional thoughts. 

It is immensely difficult for most people to proof-read their own writings. You tend to see what you thought you wrote, not what you did write. An external review is a great way to improve any writing. See, for example Ezra Pound and the drafts of The Waste Land. 

Also, you would be in far worse shape if you ignored past errors and insisted, insisted, that they were PERFECT.